You can tell in your 20s but I guess 30 and up is where the iron meets the wall and how people's life decisions effected them. I feel most of my peers still look the same but there's definitely some who have aged or look older than I imagined.
I've ran into some people and they remind me I'm not that old yet and others that make me think old age is coming soon if that makes sense. Its almost impossible for me to tell because I can't tell how old people are nowadays and I'd have to go to a reunion to get a better idea (which I don't really want to do).