Wow! this thread has exploded in 24 hours. Lots of good posts and different perspectives.
Here in the greater Phoenix area, we have lots of MLB teams playing spring training here. Lots of scouts in the area. Jake Barrett (3rd round, 99th overall pick), pitched against our HS team twice (we split). When he was throwing, there were always at least 10-20 scouts in the stands. Our #1 will attend ASU in the fall and was a 40th round pick. He attracted lots of scouts every time he pitched. The players were always playing in front of scouts it seemed.
Our unknown transfer came in from Arkansas. It took one practice for him to get noticed. He's a 6'2" RHP who sits in the 86-88 range. For the first few weeks of the season, he started and threw 3-5 and my son follwed him. Arizona State was scouting our Arkansas friend one evening and when he was done, they went over to our coach, who spoke for a few minutes with the college scouts. My son went out to start the next inning. They watched the first batter from the side and then went back into the stands behind home plate and started gunning and charting. When I say 'they will find you' I guess that's what I'm talking about. My son got a big head start that day.
Of course not everyone has that kind of access. You have to make it by going to a Perfect Game or other 'classic' to get someone to see you. This will put jr on the radar and then nature takes its course.
With that said, are there kids throwing 150 innings as 8U? Yes there are, but those kids might not be around for 12U ball. Clearly, this is wrong.
Play coach pitch, hit .975, have some fun. When your son turns 9-10, play some LL kid pitch, get some lessons, see what he's got. Start club ball around 11 or 12... Grow from there. Take time off in the off season, etc.
Can a kid who's 15 and hasn't thrown an inning become a MLB pitcher? Yes. They all develop at their own rate and speed. Great athletes develop differently. A great athlete with great hand/eye/speed/reflexes, etc can pass the common kid like a lightening bolt and there is nothing any lesson can do about that.
Watch a replay of Usain Bolt. Humans don't do run 9.38 in 100m or 19.19 in the 200m. It just isn't done. Yet it was. He could go 9.2 and sub 19 before he's done. An old track coach told me sprinters are born, distance runners are made.
These days, all athletes are born and then made.
For baseball, it's all different depending on kid/location/opportunity/HS program/options available/instruction available, etc.
In the end, if they have talent, develop it and get noticed, somebody will call. 2 out of those 3 require effort beyond the field.