Here is Proust on adolescence (from the Modern Library edition of Within A Budding Grove):
"But the characteristic feature of the ridiculous age I was going through --awkward indeed but by no means infertile-- is that we do not consult our intelligence and that the most trivial attributes of other people seem to us to form an inseparable part of their personality. In a world thronged with monsters and with gods, we know little peace of mind. There is hardly a single action we perform in that phase which we would not give anything, in later life, to be able to annul. Whereas what we ought to regret is that we no longer posses the spontaneity which made us perform them. In later life we look at things in a more practical way, in full conformity with the rest of society, but adolescence is the only period in which we learn anything."
I recognize myself in there, especially the bit about giving anything to annul our past mistakes, though in my case that desire continues up to the present day. I regret pretty much everything I have ever done.