Speaking only for myself, but finally teaching myself how to avoid that kind of messianic thinking has been a considerable benefit to me in my old age.
How do you know you just didn't get bored with your old ideals and just moved onto some new thing? "I'm older than you" has never been a convincing argument, and the fact that you're bringing it up seems to indicate that you think it should have some moral weight that it does not.
Maybe when I grow older (I am just 21), I will become more "realistic" and altruism will not make me happy. This is a sad possibility.
But I have met quite a few people much, much older than I am (40-70 years old) who are happy changing the world for the better.
You only live once, why not set the bar high? That is how they approach the world. Having that sort of "mission" (because it really is a mission, not just a goal or a dream) makes their lives fulfilling and quite inspiring.
They think about what they will do when they grow up, when they are 60, or 90.
To me, they have much more youth to them than some of my friends who have already lost their dreams.
I have no idea, because I am so young still, but seeing people like that makes me think: age really is just a number.
Speaking only for myself, but finally teaching myself how to avoid that kind of messianic thinking has been a considerable benefit to me in my old age.