Is it a personality thing?
I'm not sure. I have more people in my phone that are my age, but the ones I consistently call, text, and hang out with are the older ones. No idea exactly why. I think it's that I'm closer to my older friends in terms of where I am in my life. I'm ambitious and want to learn and grow and accomplish everything I possibly can. A lot of people my age, especially those I knew from high school, are still in the party phase of their life. A brief look at my feed from people who went to my high school reveals that 3 were day drinking Thursday, and one when to a hookah lounge Friday and puked in the bushes.
High school is high school. There's no filter and it's only maybe 2000 people at most. Were you the oldest person you hung around in high school? 20 and 21 year olds can be pretty immature. Also, maybe you need to take some of the names out of your phone and maybe it's a bad idea to add new ones unless they really deserve a space in your contacts.