> I think it is more you start out trying to fix the worlds problems,

I can't think of that many young twenty-somethings who want to fix the worlds problems. If anything, younger people are more focused on themselves [1] that people after the 30-35 yo mark.

[1] That's not an accusation or anything. It's natural that, when you're young, you need to figure out who you are and what you want out of life, and that requires a lot of focusing on oneself. Not to mention just growing into being an actual adult is also not easy and requires a lot of trial, error and introspection - something that is not as required when one is older and more formed already.

Speaking as someone not totally young, and (hopefully) not totally un-smart, I think its true one mellows out with age, also you can "be the change you want to see". That to me is the only thing we can do. If we all do our bit we can make things a bit better. Some people can make a disproportionately large positive difference. Its never too late to do that.

"going through the motions" is a phrase associated with sadness, not happiness. That's not at all what OP was describing.

EDIT: rephrased to not be snarky