I can remember doing it as young as 5 & 6 years of age ... My teenage years were particularly intense, and this is when I became aware of what I was doing even though I still didn't understand the why ... I quit doing it sometime in my 20's ... But when I escaped that hell hole I grew up in and started working on my issues at 33.5 years of age, it returned with a vengeance ... It finally settled back down a bit in my early 40's ... However, I find I still engage in it a bit - especially when extremely stressed - and, I'm now in my 50's.
Mine stemmed directly from the physical, emotional and sexual abuse I was subjected to on a damn near daily basis from the time I was 3 years old. Many of the abuses often left injuries on my body that I would aggravate even more and not allow to heal.
For the record ... I also include eating disorders, addictive disorders, suicidal ideation (and/or attempts), and other forms of self abuse (physical / emotional / sexual / spiritual) in this category ... To me, anything we do to harm ourselves is a form of "Self Injury" ... Even though others may not necessarily agree.