Laudator Temporis Acti

Cassiodorus, Variae 1.24.3 (tr. Thomas Hodgkin):

Bring forth your young men for the discipline of Mars. Let them see you do deeds which they may love to tell of to their children. For an art not learned in youth is an art missing in our riper years. The very hawk, whose food is plunder, thrusts her still weak and tender young ones out of the nest, that they may not become accustomed to soft repose. She strikes the lingerers with her wings; she forces her callow young to fly, that they may prove to be such in the future as her maternal fondness can be proud of. Do you therefore, lofty by nature, and stimulated yet more by the love of fame, study to leave such sons behind you as your fathers have left in leaving you.

producite iuvenes vestros in Martiam disciplinam: sub vobis videant, quod posteris referre contendant. nam quod in iuventute non discitur, in matura aetate nescitur. accipitres ipsi, quorum victus semper ex praeda est, fetus suos novitate marcentes nidis proturbant, ne molle otium consuescant: alis verberant immorantes, cogunt pullos teneros ad volatum, ut tales debeant existere, de quibus possit pietas materna praesumere. vos autem, quos et natura erigit at amor opinionis exacuit, studete tales filios relinquere, quales vos patres vestros constat habuisse.

The saying "Quod in iuventute non discitur, in matura aetate nescitur" (What isn't learned when we're young isn't known when we're grown) is number 2000 in Renzo Tosi, Dictionnaire des sentences latines et grecques, tr. Rebecca Lenoir (Grenoble: Jérôme Millon, 2010), p. 1456. Tosi compares the German "Was Hänschen nicht lernt, lernt Hans nimmermehr," but not our "You can't teach an old dog new tricks."