Individual qualities and quantiles that a youngster has should determine if and when pitching, at any level and intensity, is warranted. In fact, we've seen here from video requests, such a wide array of those with, and without, a skillset that can be reasonably worked with. Some are natural athletes, while at the other end of the spectrum, we have the late bloomer.
Relating and enjoying what this sport can be, for me anyway, is watching Tee ball. There, one can see the makings of good coordination, and then, less so. Follow up the ladder in age and competition, and same-mo same-mo.
I know this is a broad-brush response, but it fits so many individuals.
I've said this a million times to my guys when I assumed responsibilities - "if this was easy, anybody could do it."
For those who are fortunate enough to "have it", the age that responds to reasonable maturity, a willingness to be counted on, and the ability to set priorities, I think, is a good age for just about anything.
Just a quick story:
I had two brothers and another youngster who lived a few streets over from me who played hockey for most of the time I knew them. I believe at about 6th grade I noticed the two brothers being driven by their mom to the local hockey rink, every morning before school at around 5am. It was winter and I shutdown during this time. But every morning when I stepped out to grab my morning paper around 5am - there they were going to the rink for hockey drills. I live in Western New Massachusetts and hockey is really big here.
The youngster who didn't go to the rink was no less a player - in fact he was one of the best in our tri-city area. He had all the tools, but none of the attendance at the rink like so many others.
All three ended up playing varsity hockey for the same high school later on. All three got pro offers after graduation and all three went on to play professional hockey.
I would be amiss if I didn't say others attended the same 5am sessions, and others could have been introduced to a higher level of play - but they weren't for whatever reason(s).
So, in my opinion, with youth baseball, velocity is so overrated at the youth level , right up into high school. The lack of competent coaching skills particular to pitching, coupled with the lack of any reasonable field maintenance, - I'd focus more on a youngster controlling the ball, rather than having the ball control them. And then there's the pipe dreams of parents and family members with their visions of college and possibly pro ball, all thrown in for good measure.