Jean-Pierre Torrell, O.P., is the contemporary expert in St. Thomas’s biography. He writes, on Saint Thomas Aquinas: The Person and His Work p. 1:
The date of Thomas’s birth has been calculated approximately on the basis of the date of his death. His first biographer informs us that he died the morning of 7 March 1274, in his forty-ninth year.1 This may mean that Thomas had already lived more than forty-eight years, but had not reached his forty-ninth birthday. But the same author also adds: “Having finished the forty-ninth year of his life, he began in his fiftieth the jubilee of eternal glory.” His birth, therefore, should be placed in 1225.
Bernard Gui, who wrote several years later, gives parallel confirmation that Thomas died when he had completed forty-nine years and begun his fiftieth year.2 In a slightly earlier text, Tolomeo of Lucca echoes an uncertainty: “He died at the age of fifty, but some say forty-eight.”3 There seems to be agreement today on 1224/25, but other secondary sources do not allow the complete exclusion of 1226 or even 1227.4
Ystoria 65, p. 395 (Tocco 65, p. 138). [English transl. p. 209]
Gui 39, p. 205. It may nevertheless be asked if Gui does not take too seriously the mystical speculations of Tocco, for whom, after seven sabbatical years (7×7 = 49), Thomas would finally have entered into eternal rest for his fiftieth year.
Tolomeo XXIII 10: “Obiit autem L [50] vitae suae; alii vero dicunt XLVIII [48].”
Thus Tugwell (p. 201) chooses 1226 (C. Vansteenkiste, RLT [Rassegna di letteratura tomistica (new series of the “Bulletin thomiste”), Naples] 24 [1991] 11, points out the problems with this position); Scandone (pp. 8–9) presents the different source data; cf. WN [A. Walz, Saint Thomas d'Aquin, French adaptation by P. Novarina, Philosophes médiévaux 5, Louvain-Paris, 1962.], p. 16, and P. Mandonnet’s research, which is still sound in its conclusions if not always in its details, “Date de naissance de S. Thomas d'Aquin,” RT [Revue thomiste] 22 (1914) 652–64.
There’s also the (apocryphal / oral-only?) tradition that he died short of 50 years old, because Plato said a man must be at least 50 years old to attain wisdom (Republic bk. 7, toward the end*); and St. Thomas, being so wise, thanks to divine illumination, was able to attain it before 50.
*“when they have reached fifty years of age, then let those who still survive and have distinguished themselves in every action of their lives and in every branch of knowledge come at last to their consummation”