Saint Athanasius of Alexandria (/ˌæθəˈneɪʃəs/; Greek: Ἀθανάσιος Ἀλεξανδρείας, Athanásios Alexandrías; c. 296–298 – 2 May 373), also called Athanasius the Great, Athanasius the Confessor or, primarily in the Coptic Orthodox Church, Athanasius the Apostolic, was the twentieth bishop of Alexandria (as Athanasius I). His episcopate lasted 45 years (c. 8 June 328 – 2 May 373), of which over 17 were spent in five exiles ordered by four different Roman emperors. Athanasius is a renowned Christian theologian, a Church Father, the chief defender of Trinitarianism against Arianism, and a noted Egyptian leader of the fourth century.
How old was Saint Athanasius of Alexandria when he became known as the chief defender of Trinitarianism against Arianism?