King Edward III of England married Philippa of Hainault in York on 24 or 25 January 1328, just about exactly on his parents' twentieth wedding anniversary (Edward II and Isabella of France married in Boulogne on 25 January 1308). The young king was fifteen at the time of his wedding, born on 13 November 1312. Philippa's age and date of birth are unknown for certain; here's a post about it, and about her siblings and their birth order.
The first thing to note is that we do not know Philippa's date of birth for certain. The date often ascribed to her online, including on her Wiki page, is 24 June 1314, but this is merely the date of birth of her eldest sister Margareta wrongly assigned to Philippa coupled with an assumption (a reasonable one, admittedly) that she was born in 1314. I repeat: there is no record of Philippa of Hainault's exact, or even approximate, date of birth, so if you see someone claiming she was born on 24 June 1314, either they've stolen a march on all other historians by finding a contemporary document which records the date (highly unlikely) or they haven't done their research properly (infinitely more likely). Neither was Philippa the second daughter of her parents, as is usually stated. This is based on a statement by the chronicler Jean Froissart, who gives the names of the sisters as Margareta, Philippa, Johanna and Isabella, in that order (in the original spelling, Margerite, Phelippe, Jehanne et Ysabiel). Froissart was wrong; Philippa was the third or fourth daughter of her parents. She also had one brother, Willem, as well as several other siblings who died in infancy and whom I won't deal with here.
Before examining the evidence for Philippa's date of birth and the birth order of her and her sisters, I'm going to take a quick look at her background. Philippa's father was Willem, born in about 1286 or 1287 and count of Hainault and Holland in the Low Countries. He inherited Hainault from his father, grandfather and a long family line of counts of Hainault, and Holland from his father, who had inherited it from his cousin Floris V's son Count Jan I when Jan died childless aged fifteen in 1299, having been married to Edward I and Eleanor of Castile's youngest daughter Elizabeth. Philippa's mother Jeanne de Valois was the third child and second daughter of Charles, count of Valois, brother of Philip IV of France, and his first wife Marguerite of Anjou-Naples. Charles and Marguerite's first daughter Isabelle, who married the future Duke John III of Brittany and died when she was seventeen before her husband succeeded his father, was born in 1292, and their first son King Philip VI of France in 1293. Jeanne therefore was probably born in 1294 or 1295, unless she was a twin of Isabelle or Philip. Three more children – the countess of Blois, the count of Alençon and a daughter who died in infancy – followed Jeanne before the premature death of Marguerite of Anjou-Naples (eldest daughter of Edward I's first cousin Charles, king of Naples, and Marie of Hungary), aged about twenty-six, in 1299. Philippa of Hainault, via her mother, was the niece of Philip VI of France and the great-niece of Philip IV. Her mother was the first cousin of Edward II's queen Isabella, Philip IV's daughter, which meant that Philippa and her husband Edward III were second cousins. Jeanne de Valois and Count Willem of Hainault and Holland married at Chauny in Picardy, northern France on 19 May 1305, the year after Willem succeeded his father. Jeanne cannot have been more than thirteen and was probably only ten or eleven; Willem was about eighteen.
Given Jeanne de Valois's youth at the time of her marriage in 1305, she is unlikely to have given birth much before 1310. She and Count Willem had several children, mostly daughters; establishing their exact number, birth order and the date of birth of their daughter Queen Philippa is tricky. They had a son Willem who was their only son to live past infancy and who died childless in 1345, and Margareta must have been their eldest daughter, as she inherited the counties of Hainault and Holland on the younger Willem's death. Margareta was born on 24 June 1310 or 24 June 1311. This date is based on a famous description of her by Walter Stapeldon, bishop of Exeter, when he visited Hainault in 1319 to negotiate for a marriage between Margareta and Edward of Windsor, the future Edward III of England. The girl described was not Philippa, as is often assumed, though the description has sometimes, bizarrely, been used as 'evidence' that Philippa was England's first black queen, which is absolute nonsense. Stapledon wrote that the girl he met being considered as the future king of England's bride, almost certainly Margareta, though it might also have been Sibilla (see below; Stapeldon would have done us all a big favour if he'd bothered to record the girl's name), would become nine years old at the next Nativity of John the Baptist, which is 24 June. Depending on when in 1319 he wrote this, it means that Margareta was born on or shortly before 24 June 1310 or 24 June 1311. [See Ian Mortimer, The Perfect King, pp. 403-4] 24 June or a little before was Margareta's birthday, not Philippa's.
Margareta, as she ultimately inherited the counties of Hainault and Holland on her brother's death, was certainly the eldest daughter (or in the interests of strict accuracy, let's say at least the eldest surviving daughter) of Willem and Jeanne. This is beyond doubt. The second daughter was not Philippa, however, but Johanna, as the chronicle of 'Willelmi, capellani in Brederode' states: it twice calls Johanna the secunda filia of Count Willem. [Willelmi, capellani in Brederode, postea monachi et procuratoris egmondensis chronicon, ed. Cornelis Pijnacker Hordijk, pp. 145, 244] Margareta and Johanna married in a double wedding in Cologne on 26 February 1324; Philippa wasn't betrothed to Edward of Windsor until August 1326 and married him in January 1328, which also makes it virtually certain that she was younger than Johanna. Margareta married Ludwig or Louis von Wittelsbach, duke of Bavaria and Holy Roman Emperor. She was twelve or thirteen at the time of her wedding in February 1324, depending on whether she was born in June 1310 or June 1311, and Ludwig, born in 1282, was much her senior, older than her parents, in fact. The second sister Johanna married Wilhelm, later duke of Jülich, in February 1324. It is possible, though I'm only speculating here, that Margareta and Johanna were twins, with Margareta the elder and her father's ultimate heir. If not, then Johanna must have been born in 1312 or 1313 and been only ten or eleven at marriage (her husband Wilhelm of Jülich was born in about 1300).
Count Willem of Hainault and Holland had, between 1318 and 1321, been involved in negotiations with King Edward II of England regarding the future marriage of one of Willem's daughters and Edward's son Edward of Windsor, the future Edward III. Although Edward III later married Philippa, she was not the daughter in question at this time, as is sometimes assumed. A marriage between England and Hainault was on the cards sometime before 10 December 1318, when Edward II asked Pope John XXII to issue a dispensation for his son Edward and Willem's daughter Margareta to marry; they needed one as they were second cousins, both great-grandchildren of Philip III of France and Isabel of Aragon. [Foedera 1307-27, p. 381] To complicate matters, however, there was apparently another sister, Sibilla, who on 2 November 1319 was named as the potential future bride of Edward of Windsor, not Margareta, in various letters sent by Edward II. Either Sibilla had been substituted for her sister, or this was an error by Edward II's clerk, who carelessly named the count of Hainault as Robert instead of William in the letters (the count of nearby Flanders was called Robert, so presumably he mixed the names up). [Foedera, p. 405] On 9 November 1320, in a further letter about the potential betrothal of Edward of Windsor into Hainault, Margareta was again named as the future bride, though a frustrated Edward II merely referred to 'your daughter', unnamed, in yet another letter to Count Willem about the matter on 30 March 1321. [Foedera, pp. 437, 446] Edward II was still evidently keen on the alliance well over two years after it was first proposed, but Willem had for some reason grown cold on the idea. The king of England gave up and began negotiating with King Jaime II of Aragon for a match for his son (which ultimately also proved unsuccessful). Her proposed English marriage having failed, Magareta married Ludwig von Wittelsbach in 1324.
Sybil or Sibilla could be used as a pet name for women and girls named Isabella in the fourteenth century, rather than being an independent name. Philippa of Hainault certainly had a sister named Isabella, but she must have been considerably younger than Philippa, as she married Robert of Namur, son of John I, count of Namur. Robert was born in about 1323 or 1326, and is thus unlikely to have married a girl born before 1320, an age gap which would have been considered too great given that it would have limited the number of Isabella and Robert's fertile years together. The Sibilla mentioned in English documents of 1319 may therefore have been yet another sister, and she probably died as a child or adolescent as she does not appear again in any other known source of the period and did not marry. (Unless she was simply a figment of an inattentive English clerk's imagination.) Philippa of Hainault therefore had two or three older sisters, Margareta, Johanna and perhaps Sibilla, as well as a younger sister Isabella, born in the 1320s and perhaps named after her dead older sister Sibilla and ultimately after their great-grandmother Isabel of Aragon, first wife of Philip III of France.
As mentioned above, it is possible that there was a set of twins among the sisters, perhaps Margareta and Johanna (as they married on the same day in 1324) or Margareta and Sibilla (as they were both proposed as brides for the king of England's son), with Margareta as the elder twin who inherited Hainault and Holland. But we don't and can't know that for sure. Their brother, called Willem after their father, married Johanna of Brabant, eldest child and ultimate heir of Edward II's nephew Duke John III of Brabant, but had no children with her. The younger Willem's date of birth is not known; presumably in the mid to late 1310s, or even after 1320. His wife Johanna was born in 1322. Margareta, Johanna and Philippa were the only three Hainault siblings who had children (Johanna's daughter Elisabeth of Jülich married Edward II's nephew John, earl of Kent, the posthumous son of Edward's half-brother Edmund of Woodstock).
1314 seems to me a reasonable year of birth for Philippa of Hainault, though when exactly in that year is impossible to say. Jean Froissart says she was 'on the point of becoming fourteen' when she married Edward III, but as he says that Edward was seventeen at the time when actually he was fifteen and gets the birth order of Philippa and her sisters wrong, we shouldn't necessarily place too much weight on that. Anyway, it seems likely to me that Philippa was about thirteen or fourteen at the time of her wedding in January 1328 and fifteen or sixteen when she gave birth to her eldest child Edward of Woodstock on 15 June 1330. Her youngest child Thomas of Woodstock was born on 7 January 1355, when she would have been forty or so, and she died on 15 August 1369, in her mid-fifties.