I was just thinking that lets say you take a 13-15 year old kid just starting through puberty while doing PE excersises. Do you think the combination of both would almost add exponential growth? Very interesting in my opinion.
You need to consider a very long list of characteristics:
maturity (especially mental maturity);
how is the development of the child's body (advanced for his age, or not);
and a load of genetic factors you don't know of.
First let's look at what can be positive factors:
The young age gives the child a lot a regeneration power, so the effects of the more straining exercises can be diminished because the body will heal a lot faster, also that will improves the child's ego since it will provide them with a very solid core in an area where most men are very sensitive (that sensitivity beginning at that age range).
Some bullets which may be negative:
The young age also would provide us with a spectacle of a super inflated ego and a rudeness beyond comparison, this could cause a lot of over training, which would be highly negative, also the child would be bored by not seeing any immediate results and would either drop out of the routine or over train, once again, resulting in injury.
I was considering this as well. It would seem like an ideal time, and coupled with natural growth that occurs at that age, could give great results. But younger people tend to do less research and take less caution, so I say that risk of permanent injury is much higher. It's probably better to know what your permanent size is before attempting PE. I think 18+ is the only suitable time to start PE.
FWIW, maturity doesn't magically appear at 18 so I don't know if there is a straight forward answer to this question. In the "Karate Kid" view of teaching, I kinda wish I learned how to jelq and edge rather than frantically dry masturbate as a teenager. I think I would like to have that conversation with my son when he's the right age in another decade.