There are a great number of 'older' people with scars. I have certainly met a number of people in their 40s with scars, some of whom don't mind them being visible and some who go to great lengths to hide then.
Self harm is something that seems to affect younger people more but the reason for that is that most people do get better and move on from it. There is far more evidence that supports this than supports the idea that self harmers kill themselves. Things really can get better, even if its been a long standing issue for you.