The older the girls get, the fewer stay in softball. Departures range from other sports, jobs, boyfriends, realize they don't want to work as hard as needed to improve, lack of interest, injuries, etc. Too numerous to list.
If you look at the forum for teams looking for players you can see a large number of teams in 16 & 18u looking for players and higher percentage yet looking for pitchers and catchers in those ages. And both those positions are the ones taking the most extra work and dedication to excel at. No coincidence.
I would have to agree with CARDS above about starting early on getting girls & families on board. Usually , there are a raft of tryouts come early August here in Cincy area.
Some teams are able to attract players at all times based on history and reputation. And others struggle finding players based on same criteria. Example, if you witness Team XYZ winning several tourneys at multiple ages, for several years, people assume that it is strong org and great teams to play for. Conversely, if you see Team PDQ getting the snot kicked out of them all the time, they might not be on your short list of dream orgs to join.
Best of luck.