I somehow don't think that Mario could possibly have been more than 10 years younger at the time of the original Donkey Kong.
This argument is confusing to no end. But really, I'm rather surprised that most people here seem to think that any solid theory can be created concerning this. It would be idle to pretend that the continuity wasn't screwed up by Rare and/or post-Rare Nintendo, and therefore it is only to be expected that one can't make things make perfect sense now. It's just something that can't be helped, it seems.