Bruce Lee would have disagreed
Oh ju jitsu is different, weight helps there because its all about grabs / throws.
Bruce Lee was actually strong though (I do think his feats of strength have been exaggerated quite a bit however). If you can manipulate your opponent's weight, there is very little they can do to defend against that. I mess around with some friends doing boxing and Brazilian jiujitsu, one of them is about 13 stone, and I'm about 20 or so. I can completely manipulate his weight so I can move and throw him about easily, he can barely move me at all, on top of that, chokes, holds and moves that he can do simply don't work on me because of the differences in strength. I can bare his chokes, even when he's got straight access to my neck and anything else he can do. Same goes for the instructor we were training with as well, I can bare his chokes, holds and resist things like armbars, I'm bigger and heavier than him too.
Now I know it's not entirely based on the size and weight of someone, but it definitely helps a huge amount. If you can't apply enough strength to perform a maneuver, then having all the technique in the world is useless. I think the main situation where a weaker person with technique can overcome some one much stronger, is when the stronger person has absolutely no technique and is slow and cumbersome.
From my own perspective and experience, you need a good combination of strength and technique. When I started learning bits of jiujitsu, I could resist and defend easily enough, but couldn't perform anything outside of brute force "bend arm until they tap out" types of things.
I'm not sure what age is suitable to get them in to it, but I myself would love to have started this type of stuff at an early age.