I just wanted to add that my mom has issues with her knees hurting as well, although hers are probably due to the fact that she is getting up in age.
We both ride Western, although I have been venturing into the English riding for the past 3 years while I am going to grad school (which is not close to home).
Anyway, my mom swears by GLUCOSAMINE and CHONDROITIN supplements. She takes them every single day and she notices when she forgets to take them, because her knees start hurting especially when she rides. But if she takes it, no knee pain!
Glucosamine and chondroitin are two of the things that make up the cartilage in your joints that does slowly degrade with age. Taking the supplements is thought to increase production of cartilage.
So even though you are only 18, it would still be worth a shot to try the supplements. They certainly aren't going to hurt you in any way if you don't actually need them (it's actually been shown to be protective), but who knows, it might help with the pain. You can get them anywhere you can buy multivitamins. There's a few different formulas; my mom usually buys all 3 formulas and just rotates through them (takes 1 kind one day, the next kind the next day, etc).
I do agree that it would be best to see a doctor for the knee pain, but I understand completely. If I am not dying or a limb doesn't need to be cut off, I usually don't see the doctor either.
I just put up with whatever it is. (Which really is strange coming from my mouth because I am in optometry school right now to become an eye doctor.) My excuse is I have no money to see the doctor because I am a poor grad student where nothing but sleep and Vitamin C can cure!