Sources

1) Source: Special Collections: Ancient Petitions "John de Moubray (Mowbray), son and heir of John de Mowbray.""It is agreed by the council that he have his lands by the extent until he is of age, thus that he do the services due to the king for the lands as was done for his father.""[? 1327]". Reference: SC 8/176/8787 (Data from direct source)
2) Source: Special Collections: Ancient Petitions "Petitioners: John de Mowbray and Joan de Mowbray, his wife.""Nature of request: John and Joan de Mowbray state that they have been sued in the common bench by Thomas de Breuse regarding the manors of Washington, Findon and West Grinstead, but the case has been pending for over six years without judgment. They ask that the king order the case to be summoned before the peers in parliament so that judgment may be reached.""People mentioned: John de Mowbray, father of the petitioner; Thomas [de Breuse (Braose), son of Peter de Breuse; Peter de Breuse (Braose).""[1348]". External Link
http://discovery.nationalarchives.gov.uk/details/r/C9294156. Reference: SC 8/167/8311 (Data from direct source)
3) Source: Chancery: Inquisitions Post Mortem, Series I, Henry IV "Thomas Moubray (Mowbray), earl Marshal...John Moubray, his brother and heir...He held the manors of Bosham and Stoughton in fee tail by the grant of Edward II to Thomas de Brotherton, his brother, of the lands of Roger le Bygod, sometime earl of Norfolk and Marshal [CChR 1300–26, pp.205–6]. They descended to Margaret duchess of Norfolk as daughter and heir of Thomas, from her to Thomas de Moubray, duke of Norfolk, as son of Elizabeth, daughter of Margaret, and so to Thomas as son of Thomas Moubray, and now to John Moubray as brother and heir...to William de Brewosa, senior, with remainder to John de Moubray and Aline his wife and their heirs. From them they descended to John their son, John his son, to Thomas duke of Norfolk, his son and heir, to Thomas the Earl Marshal, and now to John his brother...7 messuages and 10 bovates in Epworth, Haxey, and Owston in the Isle of Axholme, which John Lord Moubray, son of John, son of Roger"Dated: 1407. External Link
https://www.british-history.ac.uk/inquis-post-mortem/vol19/pp101-120. Reference: C 137/62/76 (Data from direct source)
4) Source: Chancery: Inquisitions Post Mortem, Series I, Edward I "Matthew de Loveyn, alias de Lovayn, de Luveyn...Friton. The manor (full extent given with names of tenants), including customs called ‘Morlay,’ held of John son and heir of Roger de Mowbray, now in the king’s wardship, by service of 1/4 knight’s fee."Dated: 24 May 1302. External Link
https://www.british-history.ac.uk/inquis-post-mortem/vol4/pp54-67. Reference: C 133/105/7 (Data from direct source)
5) Source: Special Collections: Ancient Petitions "Petitioners: Aline de Mounbray (Mowbray), widow of John de Mowbray.""People mentioned: William de Brewosa (Braose); Earl of Hereford; John de Mounbray (Mowbray), late husband of the petitioner; Hugh le Despenser, the son; Lady de Burgh; John de Mounbray (Mowbray), son of the petitioner.". External Link
http://discovery.nationalarchives.gov.uk/details/r/C9294497. Reference: ha (Data from direct source)
6) Source: Special Collections: Ancient Petitions "Aline de Mountbray (Mowbray), widow of John de Mowbray, daughter and one of the heirs of William de Brewes (Braose); John de Bohoun (Bohun) of Midhurst, kinsman and heir of William de Brewes (Braose)"Dated: 1327. External Link
http://discovery.nationalarchives.gov.uk/details/r/C9209997. Reference: SC 8/157/7805 (Data from secondary evidence)
7) Source: The Mowbray Estate "John lord of Segrave; and John son of John lord of Moubray and Elizabeth his wife. 16 Aug. 23 Edw. IIIJohn de Segrave has granted to John and Elizabeth, for Elizabeth's life, a rent of 66s. 8d. from his manor of Diseworth (Leics.).". External Link
http://discovery.nationalarchives.gov.uk/details/r/54e5178c-ec3d-4f85-bfef-b54a545bf123. Reference: BCM/D/1/1/14 (Data from direct source)
8) Source: The Mowbray Estate "Whereas John de Segrave has granted to John de Moubray an annual rent of £300 from his lands in Leicestershire, and another of £200 from his lands in Huntingdonshire, and one of £100 from his lands in Warwickshire, by three charters, John de Moubray has granted that if John de Segrave enfeoffs John son of John de Moubray and Elizabeth his wife of £100 of land and rent for Elizabeth's life, and the same to them for their joint lives, the charters granting the rents of £600 a year will be void.". External Link
http://discovery.nationalarchives.gov.uk/details/r/ea0f9c6a-6581-4a64-bf60-cc1361a1a03b. Reference: BCM/D/1/1/13 (Data from direct source)
9) Source: Special Collections: Ancient Petitions "John de Moubray (Mowbray), son and heir of John de Mowbray.""It is agreed by the council that he have his lands by the extent until he is of age, thus that he do the services due to the king for the lands as was done for his father.""[? 1327]" Suggests a birth of 1310 (or later) as under age in 1327.. Reference: SC 8/176/8787 (Data from secondary evidence)