Well the legal way is for a parent or guardian purchase it for you
Any age, but I think if your under 18 then your parent account has to turn of age restrictions on things in the store.
it is 18 and over
18
Mature (17+)
beah
18+
The legal rating in America to purchase Modern Warfare 3 is 17, however, many children far under that age. It's up to you when you allow them to play it.
NO as he shouldn't be playing ether game as he is under the age
18+,Rated "M"
its an 18 (approved by game)
it is rated mature 17 and up
It will likely be mature but right now it is RP