If you do not strive in youth, in old age you'll only grieve in vain.
Tips
history
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.
usage
Always paired with 老大徒伤悲 (in old age you'll only grieve in vain). 少壮 refers to the prime of life, not just childhood.
