If a pitcher throws one legitimate pitch 90 some MPH, it means he is capable of throwing that velocity again. That is a good thing, but It doesn't mean he will average that velocity in the future, but it is possible. Truth is you can pretty much bet you weren't lucky enough to see a pitcher on the very best day of his life, or worst day for that matter. So you could assume that pitcher is capable of doing better. And with any young pitcher you can assume he will get better as well as add some velocity in the future. Gains in velocity can be very different from one pitcher to the next. We have seen very large gains in one year or even less. We have also seen very small or even no gains in one year or more. Also each pitcher is different when it comes to the timing of these increases in velocity. For some it could be a steady climb over four or five years. Some make major improvements at different ages. Some don't develop their top velocity until college years. Others have reached their limit by age 16 or 17.
What I'm trying to say is that it doesn't matter what happened with one pitcher, there is no average that means anything. Because it has no bearing on what might happen to a different pitcher. It is kind of like the average age of everyone, has no bearing on how old you are.
The thing to watch out for is that one pitch that is 3-5 mph or better than the second best reading. This is usually a case where the radar has picked up the ball off the bat. It happens quite a bit and can get parents excited and even angry when they see their son wasn't recorded at 96 mph or whatever velocity. They seem to think that really happened because they saw the radar reading. They don't consider that his highest pitch other than that one 96 was 85 mph. I'm sure most people on here understand that, but you would be surprised by how many don't.