Serious aches?!

Serious aches?

i am only 16 and i have trouble doing simple things like playing tennis or riding a bike for an extended period of time because or back and knee pain... will this only get worse?

Could be you're not easing into it enough and having muscle strain, especially if you've been sick or laid up in bed, or not used to doing these exercises. Could be arthritis or just joints rubbing together. Have you ever fallen down stairs, been in car accidents, or had other physical trauma? Could be lasting injury from that that never healed right and can go to physical therapist and or chiropractor. Could be lupus, fibromyalgia, chronic muscle fatigue, chronic fatigue syndrome, or other, some disorder not even mentioned here. You say it is for extended time. You could be overstressing muscles and joints. Just try to do it next time 15-30 minutes at a time and work your way up from there if need be for tournaments.
You really do need to see doctor if you can. Too many could be's here and we can't examine you on the computer, not even the doctors, sorry. There are also online tests and diagnoses you can take and print out for your doctor to see when you score a high score in one area so the doctor may know more what to look for too and what to rule out. If you are allowed, you can try to take ibuprofen to reduce pain and possible swelling and put on freeze-it (menthol pain reliever) from pharmacy until your appointment. Be sure to ask nurse or doctor on phone if you can do this until appointment or if it hurts too bad, go to hospital for quicker relief. If you use heating pad or shower do before freeze-it. Do not use freeze it before heating pad (burn). Research online RICE (rest, ice, compression, elevation). Do gentle stretching exercises if needed from doctors after getting appropriate rest and treatment.