At 24 and being normal weight and in the absence of any significant trauma, you shouldn't be having any problems with your knees. It's absolutely not part of the motorcycling lifestyle, as many of us who've been riding decades can attest. I don't really care what position your legs are in while you ride, you're too young for it to matter for more than about 15 minutes after any given outing. I'd strongly recommend that you go see a doc. Start with your regular GP and insist on referral to a specialist is the answers aren't to your satisfaction.
Some questions to nail down though: up and down stairs? Which is worse? Does it start with going up or down or at another time? All day or early in the morning? Late at night? Any clicking or popping? Is there any swelling? Discoloration? Point tenderness? Any sensations other than pain? Tingling/numbness/heat?
And if it's all for nought, if everyone tells you that everything is fine, try yoga. Seriously.
