ah, to be young and foolish...

Ah, to be young and foolish is a way that someone older and wiser speaks of young people after they do an immature, irresponsible or foolish thing. It is somewhat of a way they remember what it was like to be young and do foolish things without realizing it or caring much about the consequences. It is typically spoken fondly of younger people doing foolish things, rather than an insult.