Your OP title poses a question, but how your OP thread reads it's more like you criticise the so called mature connotation in videogames nowadays instead based on what some teenager players might experience or approach them as they grow older. I'm not sure how to proceed, so I'll just adress the title in the question first:
Some games are, some games aren't. There are certainly games deemed mature that aren't mature at all, save for the specific gore and violence they display. Like Barkley says, the "Mature Rating" from Pegi or ESBR doesn't really deemed the game's themes mature, just that it is a game that children shouldn't play, for one reason or another.
I think referring as to "realistic" gritty games being the only mature games is a mistake in your approach, though. How a game looks like has nothing to do with how mature it is. Braid is a videogame with a specific art-style in mind and deals with mature themes in its approach.
As for the rest of your OP, I don't know. You seem to encompass a specific kind of teenagers, so I can't really adress all those other questions.
