There is no way to determine an average....you are talking about hundreds of thousands of kids. People point to outliers for some sort of frame work....a 15 year old throwing 88 is exceptionally rare as is a guy in college throwing 74 in college and having success. Both exist but neither is anything but an outlier. The average speed of a high school junior starting pitcher can be quit different in Los Angeles vs a small town in North Dakota. So, there is no real way to determine an average....people can only look at what is directly around them.
Once a kid is approaching physical maturity (say 15 or so) one can get a decent idea of where they are. Throwing 65, you are slow, throwing 85 you are fast. How guys develop, train, grow ect all play a role. I have seen guys be soft tossers until they were half way through college and then pick up 9 mph. I have seen plenty of pitchers who were bigger and stronger and more physically mature growing up, dominating youth ball and JV ball and by their senior year of high school everyone else caught up and there were not even a starter. Some guys gain velocity on a slow and steady path, 2 mph like clock work, some make jumps and long platues, some just sort of stay where they starter.
It is interesting to talk about but I would not worry about comparing oneself to specific outliers. See where you are against the kind of standard measures if you will.