If youth knew; if age could.

[O si jeunesse scavoit,
O si vieillesse pouvoit.]

Henri Estienne (1528 or 1531-1598) French printer and classical scholar [a.k.a. Henricus Stephanus]

Les Prémices, ou Le I livre Des Proverbes epigrammatizez [The First Fruits, or the First Book of Epigrammatized Proverbs], ch. 4, ep. 4 (1594)
    (Source)

Variants:

• If youth only knew; if only age could.
• If youth knew; if age could.
• If only youth knew; if only age could.