But these ages are no young adults anymore.
At least, I don't know how it works in America, but in here, you're a young adult if you're 16/17 years, and you're an adult if you're 18 or higher.
Well, to make him any younger in SMG2 would make him like 13 in Donkey Kong, which he obviously wasn't. And he's called Super Mario for a reason, he ages with spectacle.
Well I don't think he would be 49, and you don't know what age Mario was when DK 1981 was released, no one does except Nintendo, so you can't speculate like that.
I don't think Nintendo knows Mario's age either. It's like you creating a character. You don't really know what age he is unless you were thinking this particular age from the start.