I was recently looking at the Inquisition Post Mortem of Henry de Lacy, earl of Lincoln, who died on 5 February 1311 at the age of sixty. At the time of his death, Henry was acting as regent of England during Edward II's long and unsuccessful 1310/11 campaign against Robert Bruce in Scotland, and Edward sent his nephew Gilbert de Clare, earl of Gloucester, back to England to act as his regent instead - a heavy responsibility for a man who was not yet twenty. Henry de Lacy had been a close ally of Edward I, and tried his best to show the same loyalty to Edward II, but Edward's antics re: Piers Gaveston early on in his reign aggravated Henry beyond endurance, and he was a leading member of the opposition to the king and the earl of Cornwall. He did later come back to the king's side, however.
Henry was the son of Edmund de Lacy, earl of Lincoln, who died in 1258 when Henry was still a child, and Alesia de Saluzzo, daughter of Manfred del Vasto, marquis of Saluzzo. Alesia's sister Agnese married the English baron John, Lord Vescy (who married secondly Isabella Beaumont), and their niece Alesia married Richard Fitzalan, earl of Arundel and was the mother of Edmund, earl of Arundel: a spate of England-Saluzzo marriages in the thirteenth century, and one which ensured that two of the English earls of Edward II's reign, Henry de Lacy and Edmund Fitzalan, were half-Italian.
In 1311, Henry left as his heir his sole surviving legitimate child, Alice, his son Edmund having died in a childhood accident (another son, John, appears to have been illegitimate and Alice's half-brother, and later became one of her clerks, as Elizabeth Ashworth has discovered). Henry was also survived by his much younger second wife Joan Martin, with whom he had no children. Joan's date of birth is unknown but her brother William was probably born in 1294, and her second husband Nicholas Audley was born on 11 November 1292. Henry de Lacy, by way of comparison, was born at the beginning of the 1250s so must have been forty years Joan's senior. Alice de Lacy and her stepmother Joan Martin were ordered to be brought to Edward II on 22 March 1322, the day of Alice's husband Thomas of Lancaster's execution; Joan, who was probably no more than thirty at the time, is almost inevitably described with great pathos in modern books as Alice's 'elderly mother' by writers who haven't done their research properly.
Alice's mother was in fact Margaret Longespee, who died sometime after 22 August 1306 and before 16 June 1310, when her widower Henry de Lacy was already married to Joan Martin. Born in about 1254, Margaret was countess of Salisbury in her own right: she was the great-granddaughter and heir of William Longespee (Longsword), earl of Salisbury, an illegitimate son of Henry II. Her father and grandfather were also named William Longespee, earl of Salisbury. Her grandfather William, grandson of Henry II, was killed at the battle of Mansourah in Egypt in 1250 during Louis IX's disastrous crusade, and her father William died at the end of 1256 or beginning of 1257, when she was only two years old. Margaret Longespee's mother Maud Clifford, Alice de Lacy's maternal grandmother, was a granddaughter of Llywelyn ap Iorwerth, prince of Gwynedd, via Llywelyn's daughter Marared. After the death of her first husband William Longespee the third (d. 1256/57), Maud Clifford was abducted and forcibly married to John, Lord Giffard of Brimpsfield, Gloucestershire. Maud's younger daughter from this forced marriage, Catherine Giffard, was the mother of Nicholas Audley, who married Henry de Lacy's widow Joan Martin, as above. This means that the child of Margaret Longespee's half-sister married the widow of Margaret's widower. Ummmm.
Moving rapidly on, Alice de Lacy married Thomas of Lancaster, Edward I's nephew, in or shortly before the autumn of 1294, when she was twelve going on thirteen and he probably sixteen: Henry de Lacy's Inquisition Post Mortem says that in February/March 1311 Thomas was aged '32 and more' and 33, which would put his date of birth in 1277/78 - this fits well with the date of his parents' (Edmund of Lancaster and Blanche of Artois) wedding in early 1276. Marriage to Alice was an extremely advantageous match for Thomas, who would one day add Alice's earldoms of Lincoln and Salisbury to the three, Lancaster, Leicester and Derby, he would inherit from his father Edmund. Edward I had in 1292 (Foedera 1272-1307, p. 738) arranged a marriage for Thomas with Beatrice, daughter of Hugh, viscount of Avallon, son of Duke Hugh IV of Burgundy (d. 1272), but this marriage did not go ahead, I think because Beatrice died.
Edward II sent out a writ to his two escheators on 6 February 1311 at Berwick-on-Tweed, the day after Henry de Lacy's death - obviously the news reached him very swiftly - ordering them to take Henry's lands into his own hands*, and as usual inquisitions were taken in all the counties where Henry had held lands, to determine the extent of the lands and how he had held them (i.e. of the king in chief or from someone else), and the identity of his heir and her approximate age. Between 20 February and 11 March 1311, inquisitions were held in Lincolnshire, Staffordshire, Suffolk, Cambridgeshire, Buckinghamshire, Middlesex, Derbyshire, Nottinghamshire, Yorkshire, Berkshire, Northamptonshire, Oxfordshire, Wiltshire, Somerset, Dorset, Herefordshire, Lancashire, Cheshire and Denbigh. The jurors in all these counties correctly named Alice as Henry's heir, some also identified her as the wife of Thomas of Lancaster, and most had a stab at guessing her age. The guesses varied between 24 and 32, and hit just about every age in between. Not terribly helpful. However, the jurors of Denbigh, the de Lacys' great North Wales lordship, had better information. Alice almost certainly was born in the castle there, and the Denbigh jurors were the only ones not to guess her age but to give it precisely. On 21 February 1311, they declared that Alice was 'aged 29 on Christmas Day last', i.e. 1310, and thus was probably born on 25 December 1281 (or perhaps shortly before).
[* Calendar of Fine Rolls 1307-1319, p. 81; Calendar of Inquisitions Post Mortem 1307-1327, pp. 153-164.]
Inquisitions Post Mortem are a fantastic resource, but in a world still half a millennium away from inventing birth certificates, the information they provide about people's ages should often be taken with a pinch of salt, as the information can vary considerably. For example, Margaret Audley, only child and heir of Edward II's niece Margaret de Clare and Hugh Audley, earl of Gloucester, must have been born sometime between early 1318 and late 1322. In her mother's IPM taken from April to August 1342, Margaret was variously said to be 18 and 20 and thus born in 1322 or 1324 (impossible as her father had been imprisoned since March 1322), and in her father's, taken in November/December 1347, she was said to be either 24, 26 or 30, so born in 1323, 1321 or 1317 (her parents married on 28 April 1317). I also saw one recently, can't recall quite who it was now, where a tenant in-chief's son and heir was said to be either nine or fifteen. Theobald de Verdon, who abducted Edward II's widowed niece Elizabeth de Clare in 1316, was said in his father's IPM of 1309 to be anywhere between 22 and 30, but the Shropshire jurors give his date of birth precisely: aged 31 at the feast of the Nativity of the Blessed Mary last, so he was born on 8 September 1278 (or soon before). Aymer de Valence, earl of Pembroke, was said in his mother Joan's IPM of September/October 1307 to be anywhere between 24 and 37, which would put his year of birth between 1270 and 1283. Ummmm, helpful.