I'm nineteen, twenty in just over a month now.
I started when I was thirteen by cutting but I guess I'm always been a bit self harm inclined since I was little - I used to sit behind my bedroom door and repeatedly whack my head to calm down when I was angry or upset - I was about six then maybe?
Self harm doesn't really have an age limit, I guess I get why you're friend would say it as self harming can be seen as a "teen issue" but I think that's just the way it's been portrayed in the media and the like. We all cope the best we can, right? No matter how old we are