None. The only rating that's 18+ in the ESRB is AO and that isn't stocked hardly anywhere.
Alright, I'll play along, although I will point out that M is 17+...
Technically it was Halo 2, but I only played it for half an hour or so, so I'll go with the first one I owned instead.
Fable TLC
Age at time: 10
Not that bad imo, the violence only gets strong when you get a head shot with a bow and aside from that it's mainly mild language. It's also interesting to note that Fable 2 was the game that made me get around to finally picking up a 360. I think that the Fable series will always hold a special place in my heart, no matter how much Molyneux milks it. (Dear god, could I have worded that any more cheesily)