There is no law that forbids someone aged 13 from playing M rated games in the USA. It is perfectly legal for a parent to buy the game for their child who is under 17 and allow them to play it. Besides, I would imagine a 13 year old would want six in a game more than an advlt. While I don't mind if there are sixual innuendos in a game and such things as "fade to black" sixual encounters, I would hate it being done if it had no meaning or added nothing to the game other than six. If a quest called for it fine but writing a quest just to include it would be lame to me.
My preference would be to spend the time to make such an encounter on other more important aspects of the game.
And btw, let's not get into the "old fans" vs "new fans" debate here. It could be seen as flamebait/flaming and it's just old, tiresome and gets us nowhere in this discussion. Drop that part of it. Not all "old fans" hate this game, I know since I am an "old fan". and I rather enjoy FO3. Please don't generalize about fans of any kind since it will eventually anger someone and the moderators will have to intervene.