Muscle, tendon and ligament damage are likey effects of overtraining. Very painful, and they take forever to heal - if they ever do.
More subtle signs of overtraining can show up as lethargy, and lack of enthusiasm for training. Overtraining can be very difficult to recognize, especially when you are younger. That's why it pays to be more careful so you DON'T pay as you get older.
PS - happy birthday!