The "young man" referred to here is Augustine of Hippo (359-430), one of the Church Fathers, whose "writings were very influential in the development of Western Christianity" (wikipedia) and who has been "accepted by most scholars to be the most important figure in the ancient Western church" (Christian Classics Ethereal Library).
"As a youth Augustine lived a hedonistic lifestyle for a time, associating with young men who boasted of their sexual exploits with women and urged the inexperienced boys, like Augustine, to seek out experiences or to make up stories about experiences in order to gain acceptance and avoid ridicule. It was during this period that he uttered his famous prayer, 'Grant me chastity and continence, but not yet' (da mihi castitatem et continentiam, sed noli modo)" (wikipedia). In the year 386 he converted to Christianity and went on to live a life of celibacy. His most famous work is The City of God.