Girls throw their hardest at about 16 y/o. They mature fast and after that any speed increases are minimal.
Not true. It seems that way because most softball players in America stop playing between 18-25 (unless they're elite players) If you stop at your prime you won't see any improvement because you're not continuing to play!
Go watch masters fastpitch and tell me they're not still improving or are faster than they were at 16.
(from someone in a country where fastpitch continues on as long as the body holds up)
In Australia the peak is not 16 for our girls. We also don't run them into the ground. They have time to mature and let their bodies rest. That's why despite us having nowhere near the pool of players that America does, we still are able to be up the top rankings in the world. (we have 32,000 people playing softball. In the entire country. That includes juniors, seniors, masters. That also includes men)