I find that people take up RPGs very easily and quickly and all types of people 'get it' as soon as they are introduced to the concept.
I don't quite think you're getting what I'm trying to say.
Who is more likely to be exposed to the concept of an RPG? A fifteen year old or a thirty year old? When they are exposed to the concept, how does it occur? Is it more likely that they walk into a hobby gaming store, pick up a D&D book, and go, "Ooh, I like this, I'll go home and play this with my friends!" or is it more likely that a guild-member in WoW said, "You know, there's a game like this you can play around a table, wanna try it?"
The thirty year old is far less likely to be exposed to the concept of the RPG. They hang in different circles, they've already established their tastes and habits and leisuretime activities. And if they were exposed to the concept of the RPG, how would they most likely be exposed to it? Isn't it more likely that they'll pick it up through a new acquaintance introduced to them through their established social network than it is through a video game?
Now the teenager is going to pick up the books and approach it from the way he knows how, which is the min-maxing WoW DPS'er raid-monkey/PVP pwner. The thirty year old is going to approach it from what he is taught at the table by someone his own age and who was exposed to RPG's twenty years earlier.
They will both approach gaming at the table in two different ways, because they don't know any better. They will also both claim to be roleplaying, because they picked up a roleplaying game and they're playing it.
Now I'm not saying that all teenagers are like this or that all thirty-year olds are like this, I'm just trying to point out that we act on what we know, ie. what we're exposed to, we are influenced by. So it's not that a teenager has a shorter attention span, it's simply that they haven't been exposed to a style of play that requires more attention, and when exposed to such styles, there is an initial resistance because it goes against what they've established in their minds as being a roleplaying game.
Again, however, IME when I roleplay with a group of min-maxing teenagers, I invariably find that once over this initial resistance, they latch onto it because they realise how much fun it is. The opposite goes for 'true' roleplayers who I help min-max their characters and show them that hitting stuff is fun too.
Without this exposure, depending on how the player has been introduced to RPG's will determine how they view roleplaying and min-maxing. The two are not exclusive and can be melded quite happily and successfully, it's just a matter of exposing people to a different style of play.