'If only youthness may come back a day so I may lament to it the grayness'
If youth knew, if age could, Sigmund Freud
The phrase laments the inexperience and/or lack of wisdom of youth and the lack of energy of old age.
The phrase goes back much further; see The Oxford Dictionary of Proverbs here
Cf. H. Estienne Les Premices (1594) 173
O si la ievnesse scavoit, O si la vieillesse povvoit
translated as here
If when you’re young you only knew, if when you’re older you still could.