Tbh it probably started for me when I was very young, 2 or 3, or even earlier. I know quite exactly why as well, but I won't get into that.
A lot of the time for guys like us the problem was with the parents, particularly the mother.
It's just harder to detect, or be concerned about personality abnormalities when one is young, and social problems often don't become totally clear until the onset of puberty, so you tend to think something "happened" when you turned 11 or 12; it was the traumas which were imprinted upon your mind in infancy, which gestated silently in your childhood, being then aggravated to surface manifestation by the intense changes in both the inner physiological, and outer social environment. This is the age that most lifelong pyschotics have their onsets, for example.
Everybody deals with this, incels or not; it is the real reason adolescence is so stressful. The difference is that some people have few traumas and transition smoothly through life's stages. Other people... end up reading this post.