Pitchers typically gain about .7 mph for every inch of height and they also gain with age in addition to the height gain. The biggest gain due to age comes as they reach physical maturity. Usually those gains end at about 17 or 18 unless somebody is maturing quite late.
That doesn't mean that velocity gains are at an end. Scouts will often look at a taller pitcher without great velocity and project that as they fill out or learn to coordinate their long limbs that they'll gain velocity.
The other paths to increased velocity are hard work and improved mechanics.
It is hard to say about the LL kids. We had the kid who pitched in the LLWS US championship game in our league this year and although he was a very good pitcher for a 13yo he didn't seem to have gained much if any velocity since the LLWS. I don't know if that is because he hasn't grown much since then, is concentrating on command, or the guns at Williamsport were set a bit high. It will be interesting to see how he progresses through HS relative to some of the other pitchers who are the same age.
