A possible explanation is that the mellowing is due to the loss of acuity with age. The more likely explanation is that it's an evolutionary adaption in response to how ones function in society changes with age.
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.
A lot of that is why some young entrepreneurs do go on to actually change the world (and we definitely need those people, although it isn't always for the best). But most that think that's what they are going to do fail a few times, and become more realistic in their views of what they can accomplish.
Sometimes the smarter someone is, the more quickly they can think through the possible results and come to those conclusions that they probably aren't going to be one of the few that does succeed to change the world, and that can (to the overall point here) lead to a sense of sadness and loss, maybe without having gone through the effort and failure, even. Which is a bit sad for everyone, because some percentage of those people probably could succeed if they just got out of their own way and stopped over thinking things, also.
EDIT: rephrased to not be snarky