I’m 25, and I think of people in their twenties as in the same age group as me. But that comes with a couple qualifiers: if you’re 20, you’re not the same age group as me because you’re not old enough to drink. And if you still live at home with your parents, you’re not in the same age group as me because you’re at a completely different stage in your life than I am.
My age group is anyone over 62 or thereabouts. It goes up to whatever age my oldest friend might be. It has more to do with having been through lots of life experiences rather than music or activities.
10 years difference is nothing at my age, but at 20 I wouldn’t have fit with a 30 year old, or a 10 year old.