I think by Perfect Game 17u you are referring to the WWBA 17u, Oct. 6-9. That's in Ft. Myers, not Jupiter. The 18u is in Jupiter at the end of Oct.
The registration deadline for the 17u event has already passed, but only by a few days. If you move quickly they might bend the rules for you.
There are very few decent 15u tourneys around. Look for the WWBA 15u next July at East Cobb. In the meantime, you may have to play in 16u events for the most part. Many 15u teams enter the Junior Olympics and WWBA 16u. The BCS 16u would also work. If you're good you can be competitive, just don't expect to win it all or anything!