I don't know if you are talking about the average baseball pitcher or the average like varsity or future varsity pitcher. For example, if you take a good varsity pitcher, those numbers are probably right on for when he was each of those ages, but plenty of 16 yr olds play jv and can't break 72 or 73 mph.
Maybe something like this:
Your typical varsity pitcher, one year of fresh/soph, one year jv, two years varsity. At 14 he was probably around... 69 -71 Average, 72- 74 Excellent, 75+ outstanding (at 14 I was probably around 67-69)
15: 72-75 Average, 75-77 Excellent, 78+ outstanding (at 15 I was 70-73)
16: 75-77 average, 80-83 excellent, 85+ outstanding (at 16 right now I'm 77-80)
17: 80-83 average, 83-85 excellent, 86+ outstanding
I think in worse conferences these numbers will be even lower. I know my school league is pretty bad, and its more like this:
15: 67-69 average, 71-74 Excellent, 75+ outstanding
16: 72-74 average, 75-77 excellent, 78+ outstanding
17: 78-80 average, 80-83 excellent, 84 + outstanding
In fact, the best pitcher in my league last year sat at low 80s and he dominated. I hope to do the same this year. I think you play in a very competitive league Jacobs21, and those numbers are probably reasonable if you are talking about a relatively elite group. I think you have to specify what you mean by average. Average baseball pitcher? You haven't seen the truly low end of the scale.