Can you improve arm strength?

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.