His lame humour was his only real problems, as far as I was concerned. Everything was "inwented" in Russia! LOL??? I mean ... I understand not wanting the Pinko Commie getting cocky, but how about some irony in jokes, maybe? Or, at least have it be organic to the story it's in, rather than just thrown in to give Keonig a line to say. His Chekov wig looked like he was using it to hide candy in it, or something. But it's his identifying feature, so what the hell? Just go with it, I don't care.
His "youthfulness" was conveyed more to me as an attitude than an actuality. Regardless of his actual age, he was always the "youngest" of the bridge crew. He was like some 10 year old trapped in a 30 year old's body, or some shit. Except for his odd one or two moments in the spotlight, he was really just there to follow Kirk's orders and he always did so with the same professionalism as the senior members. And that I did like about him. As he once said of himself, "... I'm not that green."