I think age does make a difference for sure. It depends on what you have lived, and who you surround yourself with also. But often you see people become slightly bitter as life gets in their way, or things don't work out as simply as they had planned, etc. It's not easy to keep believing in your ideal and your sacrifices when things turn out to be much more complicated than you thought.
When you're young everything is possible, you're hungry for living and changing things. But how many people stay that way when they grow older? There are some and those people are very, very precious but overall it's mainly young people you see with that state of mind.
I don't know how it is in America but here in France for example, in general young people vote left, and old people right. To me that says quite a bit about idealism and age.
This said I think INFJs are pretty rational idealists. We will get proper informations and feedback about what it is we want to give or fight for, and try to have a good grasp of the situation and what we can do for it in a more realistic way than other NFs I think. My best friend is an ENFJ and he is so much more idealistic than I am. His main function is a feeling function and he doesn't seem to always grasp all the subtelties of a situation, things to him are rather black and white. Maybe it's better to be like that if you want to change the world. At least you have the will, the revolt in you and you're not going to sit on your ass and wait and pretend it's too complicated.
But by being too idealistic you might have a harder time achieving what you want too, because they people you will have to deal with won't share your point of view, and you won't understand why they can't see it when it's so obvious that it's unfair, etc. Typology here is very helpful.
In the end it's good to have idealism because it's an immense drive, but you have to be able to see that the goal and achievements are more important than the idea of sacrifice and healing the whole world. A dose of realism is necessary. Otherwise you might just achieve nothing by aiming too irrealistically high.
It's funny, Jung talks about the archetype of the sacrificed self at the end of adolescence. Teenagers romanticising self-sacrifice, etc. That fits it very well.