I could recommend some exercises, but would have to advise you that you should first talk to your doctor (If your doc doesn't know, ask him for one who's into sports medicine -- I don't know anyone in Philly that's into dance medicine.), or a physical therapist experienced in dance, etc.  A local dance company might be able to help you find such a professional.

At 18, some of the changes in things like your tendons can be achieved, but you have to be individually evaluated to make sure you know your limits, and don't go beyond them.

What has an experienced dance teacher told you?  Difficulty in "getting the heels down" can come from other sources.  And how do you know your plies are not deep enough?

Talk to some experts before you go about any serious stretching attempts.

Good luck!  DancingDoc