From the Yuefu ballad 《
长歌行》 (Han dynasty, anonymous):
百川东到海,
何时复西归?
少壮不努力,
老大徒伤悲。 'All rivers run east to the sea — when do they ever return west? If you don't strive when young, in old age you will grieve in vain.' Taught in primary-school textbooks, probably the single most quoted line of youth-exhortation in Chinese.