That can differ. Somewhere in old threads, a few of them in fact, there is a chart showing male developement. Someone may find that and post it. Or you could search. That gives you a good example of male developement.
What is the average penis size for someone that is pre pubertal?
Just think for a moment how silly that question is. A new born baby and a 12 or 13 year old boy can come into that category, so you are asking what the average penis size is for someone between the ages of 0 and maybe 12/13?
And are we talking erect sizes? You really think people try to get prepuberty boys erect so they can measure them?