My take on this applies to just about any field with regards to entrepreneurship. A successful entrepreneur doesn't emerge until the person 'gets it'. What I mean by 'gets it' is an esoteric combination of these factors:
-realization that life is not a dress rehearsal
-determination to be successful in their own right, and to not spend the rest of their lives asking other people for permission to trade their precious time for wages
-awakening that most barriers to success are mental
-development of absolute belief that they alone are responsible for how they feel, how they act, and their success or failure at anything.
The big difference between this and other fields is the lower physical barriers to entry for young people. There are no licences required, no physical assets to acquire, no permission needed to build software. On top of that there is a positive feedback loop where other people are willing to listen to a young software entrepreneur, because young software entrepreneurs have had success in the past. If other industries had these low barriers to entry, I would expect to find young successful people across other industries.
The age distribution I think is just the varying age at which people 'get it'. For some young people, they come out of the gates with this belief system, either innate or cultivated by their parents. For others, it requires maybe starting a family and accepting the responsibility of the same, maybe getting fired from a job, maybe a mid-life crisis or a health scare. The reasons are varied but the outcome is similar - a shifting of focus onto what is important. Once the shift of focus happens and clear thinking emerges, the entrepreneur can emerge - at any age.