Absolutely can improve it, and at about any age.
We are seeing amazing results from places like Driveline for MLB pitchers of all sorts.
It's obviously a different motion and ball, but if players are picking up 2-4 MPH on their pitches I think a lot of those techniques could be cross-trained for QBs. A little extra zip and length on throws could take some more fringe guys to a new level--as long as they don't have to sacrifice accuracy.