Blanche of Lancaster married John of Gaunt, the third (surviving) son of Edward III and Philippa of Hainault, in Reading on 19 May 1359 in the presence of three kings: Edward III and the captive kings of France and Scotland, John II and David II. Of Blanche and John's children, three survived into adulthood: Philippa, queen of Portugal, born ten months after her parents' wedding on 31 March 1360; Elizabeth, duchess of Exeter, born probably in early 1363; and Henry IV, king of England, born on 15 April (Maundy Thursday) 1367, twelve days after his father, uncle Edward of Woodstock, prince of Wales and Aquitaine, and King Pedro of Castile defeated Pedro's half-brother Enrique of Trastamara at the battle of Najera in northern Spain. Duchess Blanche died young, on 12 September 1368. But how young?
Blanche was the younger of the two daughters of Henry of Grosmont, first duke of Lancaster (b. c. 1310, d. 23 March 1361) and Isabella Beaumont, one of the daughters of Henry, Lord Beaumont, titular earl of Buchan. Blanche's older sister Maud married Queen Philippa's nephew William, duke of Lower Bavaria and count of Hainault and Holland, who went insane in 1357, and although she outlived her father, it was only by a year: Maud of Lancaster died on 10 April 1362, childless and in her early twenties. The vast Lancastrian inheritance thus passed entirely to Blanche and John of Gaunt. Maud was presumably named after Henry of Grosmont's mother Maud Chaworth, and Blanche after his paternal grandmother Blanche of Artois, queen of Navarre and countess of Lancaster.
The only real source we have for the ages of Henry of Grosmont's daughters is his Inquisition Post Mortem, which was ordered on 25 March 1361 two days after his death and can now be found in Calendar of Inquisitions Post Mortem 1361-1365, pp. 92-116. There are one or two other sources, but they're unreliable: the chronicler Jean Froissart says that Blanche was about twenty-two when she died in 1368, but frankly I wouldn't trust Froissart on such matters, and if Blanche was only twenty-two in 1368, that would mean she gave birth to her first child when she was barely even fourteen. Blanche's date of birth is currently given on Wikipedia as 25 March 1345, and her older sister Maud's as 4 April 1339. This would make Blanche fourteen at the time of her wedding in May 1359 and just turned fifteen six days before she gave birth to her first child on 31 March 1360, which is certainly possible - Edward II's elder daughter Eleanor of Woodstock gave birth to her first son in May 1333 the month before her fifteenth birthday - but is it true?
Jurors in different counties in Duke Henry's IPM give different ages for Maud and Blanche of Lancaster. Going through Henry's IPM, the Leicestershire and Warwickshire jurors said that in May 1361 the two women were respectively '22 years and more and 19 years and more', which would put their dates of birth as about 1339 and 1342. The Dorset jurors said that Maud was 23 and Blanche was '18 and more', so born in 1338 and c. 1343. Oxfordshire said that they were 26 and 21 in 1361, so born in 1335 and 1340, and the Hampshire, Wiltshire and Berkshire jurors agreed. Derbyshire, the first jurors to give specific dates rather than just ages, said on 4 May 1361 that Maud was 'aged 21 on the feast of St Ambrose last', and Blanche was 'aged 19 years at the feast of the Annunciation last'. This would put Maud's date of birth on or around 4 April 1340, and Blanche's on 25 March 1342 or thereabouts. Giving the saints' days doesn't necessarily mean that the women were born exactly on those days, though it might, but that it was the nearest feast day to their actual birthday. Yorkshire and Northumberland said that Blanche was 18 (so born in about 1343) and was certainly Henry's heir, but that as Maud had married abroad and had not returned to England, they didn't know whether she was still alive or had a child or not, and also failed to give her age. Lancashire said that Maud and Blanche were 22 and 18, so born in about 1339 and 1343. The jurors of Huntingdonshire copped out and just said that the two women were 'of full age', as did Gloucestershire, Herefordshire and the Welsh March. Rutland said they were 20 and 17, so born in about 1341 and 1344, the youngest ages given so far, though the Northamptonshire jurors immediately topped that and said the two women were 18 and '16 years and more' (born 1343 and 1345). Surrey seemed to agree with this. Lincolnshire said that they were 20 and more and 18 and more, so born in 1341 and 1343, and added that they didn't know if Maud was still alive or had children or not. The Staffordshire jurors on 6 May 1361 were the second after Derbyshire to give specific dates of birth: Maud was 21 on the feast of St Ambrose last, and Blanche was 19 on the feast of the Annunciation last, which gives us 4 April 1340 and 25 March 1342. Finally, Devon said that they were 26 and 21, so born in about 1335 and 1340.
Phew! And so, we see an all too typical spread of possible ages given in a fourteenth-century Inquisition Post Mortem, and the difficulty of determining people's correct date of birth. My absolute favourite example is Aymer de Valence, earl of Pembroke (d. 1324), who according to his mother's IPM of 1307 was born anywhere between 1270 and 1283. Mmmmmm. Helpful. Maud of Lancaster, according to the above, might have been born any time between 1335 and 1343, and Blanche, duchess of Lancaster, between 1340 and 1345. As stated above, the Wikipedia page for Maud of Lancaster gives her date of birth as 4 April 1339, which must be based on someone reading her father's IPM and seeing that two sets of jurors gave her age as '21 at the feast of St Ambrose last', but failing to spot that the jurors met and recorded their findings in early May 1361, not in late March after Henry died, and that another feast of St Ambrose had passed between Henry's death and the jurors' session. The last feast of St Ambrose therefore was 4 April 1361, not 4 April 1360, and Maud's birth year would seem to be 1340, not 1339. Blanche's Wikipedia page says that she was born on 25 March 1345, though admits there is some dispute about this and also gives the remarkably late 1347 as a possibility, which means that Blanche would have borne her first child when she was barely even thirteen. In fact, none of the jurors specifically say that she was born on 25 March 1345, though two say that she was born on 25 March 1342, and note that the jurors of only one or maybe two counties out of the almost twenty appointed to determine her age thought that Blanche was as young as sixteen in 1361, and four thought she might be twenty-one, hence born in 1340. None claimed that she was as young as fourteen, so we can put the notion that she was born in 1347 to rest. Thank goodness for that. It does sometimes happen that researchers look at the evidence of IPMs, but misinterpret it: for example, seeing that the IPM of John, earl of Kent in late December 1352 says that his sister and heir Joan of Kent (Edward II's niece and Richard II's mother) was born either on 29 September 1326 or 29 September 1327, but miscalculating it and stating that she was born on 29 September 1328, an error repeated so often that the latter date is now almost universally, though wrongly, stated to be Joan's date of birth.
In my opinion, Blanche, duchess of Lancaster, was born on or about 25 March 1342, and her sister Maud on or about 4 April 1340. These are the only two specific dates given in their father's IPM, by the jurors of Derbyshire and Staffordshire, and they both agree exactly with each other. Jurors in other counties obviously did not have access to this information, and just made their best guesses. This would make Duchess Blanche seventeen at marriage and just turned eighteen when she bore her first child Philippa, which I think is more plausible than her just turning fifteen. Maud of Lancaster died, sadly, just past her twenty-second birthday, and Blanche at twenty-six, when her youngest child, the future Henry IV, was not even eighteen months old.