I think it's certainly to do with both of those aspects, but more importantly it's to do with social and biological sophistication which means that certain things cannot be dealt with specifically without scaling down on intensity in some areas, even in 'realistic' horror where I think today's young people are possibly much more resilient than I was at that age. In fact because of that I'd say gritty reality ought to be played down rather than up because these days they don't need that laid on the line to get what's happening...
For my own 'proper' writing I'd say young adults could probably cope with 'adult themes' but a friend who's pretty broad-minded was appalled that I could even think a section on infanticide and desecration of corpses (this was fantasy BTW

) could remain in a PG13 area...

National tolerances are tricky too - in general the farther east you travel the higher it goes in terms of relationships - 'naked' in some quarters means you've still got at least one layer of undergarments on!
Blood and gore is different and in a lot of ways I'd say the average young gamer would cope with a lot worse things than I would now, at the tender age of 52
