Hello Stephen East,
Lifting is fine when done properly and safely.
You want to start out with proper form first, using light weights.
When I was a young teenager, I learned much from the enjoyable weight lifting club in school. Do you have access to such a program?
Being small for your age, as you mention, is only a very general guide. You may want to check with your doctor to see how you are doing on your personal growth chart; that is what matters. While at the doctor, let them know about your activity and joint pain. Have you spoken to your parents/guardians about this?
We all come in different shapes and sizes which are healthy.
Please click on “Add Comment” at the end of my response with any more questions and/or thoughts.
Good luck to you and keep up the motivation…safely, please.