I disagree with the statment about a 12 year-old beating a 20 year-old, even very skilled vs. little skill. If you meant a 12 year-old vs. a teenager then my opinion may be similar to yours, but I feel that the former statement could only happen in extreme cases. When I became an adult, my gain in strength was tremendous compared to when I was a teenager (could someone back me up on this, because I can only speak from personal experience and I don’t have any scientific studies to cite off hand 
)
When I work (spar, etc.) with teenagers, I have never felt at a disadvantage against even the most skilled of them, because they honestly lacked the power that I could generate with my less skilled technique (and I wouldn’t even consider myself all that strong). I have been caught by surprise a few times by speed and a flurry of attacks (I’m a sucker for fast combos 
), but I’ve never been “on the ropes” as a result.
This question is as obvious as “Muay Thai vs. Kung Fu” 
Maybe some people here can report different experiences from mine, but I have yet to see “3 Ninjas” happen in real life.