In the ancient Roman calendar of the feasts of the martyrs (Depositio Martyrum), incorporated into the collection of Furius Dionysius Philocalus, dating from 354 and often reprinted, e.g. in Ruinart [Acta Sincera Martyrum (ed. Ratisbon, 1859), 63 sqq.], her feast is assigned to 21 January, to which is added a detail as to the name of the road (Via Nomentana) near which her grave was located. The earliest sacramentaries give the same date for her feast, and it is on this day that the Latin Church even now keeps her memory sacred.
Since the close of the fourth century the Fathers of the Church and Christian poets have sung her praises and extolled her virginity and heroism under torture. It is clear, however, from the diversity in the earliest accounts that there was extant at the end of the fourth century no accurate and reliable narrative, at least in writing, concerning the details of her martyrdom. On one point only is there mutual agreement, viz., the youth of the Christian heroine. St. Ambrose gives her age as twelve (De Virginibus, I, 2; P.L., XVI, 200-202: Haec duodecim annorum martyrium fecisse traditur), St. Augustine as thirteen (Agnes puella tredecim annorum; Sermo cclxxiii, 6, P.L., XXXVIII, 1251), which harmonizes well with the words of Prudentius: Aiunt jugali vix habilem toro (Peristephanon, Hymn xiv, 10 in Ruinart, Act. Sinc., ed cit. 486). Damasus depicts her as hastening to martyrdom from the lap of her mother or nurse (Nutricis gremium subito liquisse puella; in St. Agneten, 3, ed. Ihm, Damasi epigrammata, Leipzig, 1895, 43, n. 40). We have no reason whatever for doubting this tradition. It indeed explains very well the renown of the youthful martyr.