revising beliefs

Recently I once again realised that I'm invariably getting older... - ok nothing new, and I admit I don't really mind (- yet?)
But there was one characteristic of a growing up person which keeps striking me as I observe it on some people around me:

They start becoming serious. Not in the 'no fun' way of mentality. But rather in a very fascinating way: they start to link their actions to their beliefs and convictions.

Maybe at first this seems trivial - they probably were doing this all the time one might think. But in fact this doesn't seem to be the case. Somehow very often people - especially when they are still young - have very strong (sometimes unreflected) opinions and expectations on many things and other people; without, however, living up to the standards they set. So when growing up, some of them start to revise these standards, to shape and correct their childhood beliefs, and to follow them themselves.

This process seriously intrigues me, as all of a sudden people take on the responsibility for their lifes and themselves; they are no longer just a passive product of what others imposed on them. Instead, they become active and face a challenging task, as if they were woken up from sleep, ready to change the world.