0. At that age, you have your gender, race, background, genetic disorders, basic appearance, biological parents, their initial wealthiness, and even your degree of civil liberties determined for you. At no other time in your life can you truly change any of those.

For most, I believe the early teens (adolescence) for obvious reasons. Beyond that, the elder years can be difficult if/when health problems begin to occur and quality of life is diminished.

30, that when most adults realised that they are old and began having a huge desire to fell young again.

Little more clarification is the least hard to do.