proverb/si ʒœ.nɛs sa.vɛ | si vjɛ.jɛs pu.vɛ/
youth is wasted on the young, you can't put an old head on young shoulders (it's too bad that young people lack the experience and wisdom which only come with age; at the same time, it's a pity that old people lack the physical wherewithal to take full advantage of their experience and wisdom)
Literally, “if [only] youth knew, if [only] old age could”.