21-30 is an important decade but then they all are, for different reasons.
Part of the process in that particular decade - for men, anyway - is you lose that sense of invincibility. At some point not 'everything' is still in front of you and, if you have children, the notion of responsibility for others gets a turbo boost. Also, as others have mentioned, you start to come to terms with the extent of your ignorance and even try and do something about it.
Further, in a conventional life, it's the career defining decade; by the end of it there's a good chance you'll know your field and even have a sense of how far you might progress.
Perhaps 'experience' is something else again.