Friends don't have to be in the same age range, but generally you have more in common with people your own age because you've all had similar experiences in terms of going through the same history and pop culture. I'm in my mid-40s. If I'm in an antique store or flea market with a 20-year-old and I point out an Atari 2600 Missile Command cartridge, they likely won't know what it is. If I show it to people my age, it brings up memories and suddenly we can talk about how we used to play our Ataris.