Yes you can still join, but you wont be terribly behind. If you have been in JROTC for at least 2 years you actually get a break because of the training you received in JROTC. You will still have to do the same work as everyone else, but your time in grade is 1 month, instead of 2 months like everyone else. Of course all good things have an end, the time in grade thing only takes you up to the mitchell (2dLt).
Regardless, joining at your age isn't that bad.