There will be a handful of 12's throw 75-76 every year across the country.
If you throw 68-72, you're probably in the top 1%.
If you throw 60-65, you will overpower most of your fellow Little Leaguers.
55-58 is a typical top pitcher on a LL team.
50-55 is B/B+ LL caliber speed. You can still be successful if you PITCH, i.e., change speeds, don't walk people. Or, if you're lefty!
45-50 is where just about anybody can throw.
Below 45, either you need to grow some or perhaps pitching is not for you.