I was 15 when I first started. I did it constantly until 17, then I had periods where I didn't do it and I thought I was recovered. I was able to not cut myself for years, but then, for some stressful events, I turned to my old habits. Last time I hurted myself was 2 weeks ago and I'm 22 now. When I was a teenager and I read articles about self harm, there were people who said "it's a thing only teenagers do, then when you'll be adult you will be able to cope with your emotions and problems". I thought that maybe I would outgrow from this.
But now I still think about it. I think it's different from everyone. But I think there are plenty of adults who self harm. Plus self harm can take many forms, not only hurting your skin. A common myth is that SH is only present in teenagers. And this is not true. It's not like you wake up one day, at 20, and you say "I'm an adult, no more self harm!". Its an addiction, your body and mind are addicted to this coping strategy, recovering from this is a long road with ups and downs, and relapses too.