I think they're trying to use it as a selling point...
There
were 13 year olds playing
UO, back when it was mature... But attitudes overall were still more mature.
So, it may have an effect on the number of tards running around, but conversely, the promise of mature violence may attract some.
Ultimately, I think it's more a sales line.
When you're reading something about it, and getting deja-vu every other line, because they keep plugging the mature rating... *sigh*