I agree completely:weightlift, weightlift, weightlift. Even if you're under 20. In years of mogul skiing competition, I have seen more blown knees landing jumps than I can count and most of these are kids 14-24. The ones who train in the gym the most and are the strongest definately go the longest without knee injuries.
I would say go to the doctor if it still bothers you.
However, I will admit, that I usually choose not to go to the doctor if I think he's going to tell me not to ski/kayak/climb/do what I want. If I don't go, he can't tell me not to have fun. That's a personal choice though