If we speak technically, 17 year olds and older should be the only people playing any of the Halo games, since they're rated Mature.
But screw that. What I personally think is the age of 12-14 is when kids should start playing games above the rating Teen. If the kid is pretty mature, more near the 12 year side of things. If the kid isn't mature quite yet, then 14.
I believe I was around 12 when I started regularly playing violent games, and I'm somewhat not effed up in the head.
Overall, just do whatever you parents say. Most of the time parents are right.