I'll say this much, while I know I wouldn't let my little siblings play it as is (I don't want them to be as interested in war as I), I would allow them probably around sixteen to play when theyre a bit more mature... With of course, blood and gore toggled off... Which is an option.
Once again, as mentioned above, it's really to the discretion of a parent or guardian. I know some who let their 13 year old play GTA (not exactly my cup of tea... Or anybody who's that age's), and some who strictly believe violent games should only be played when you can drive yourself to buy the game (exaggerated a bit but you should understand).
I've never played the game myself, but I have seen it played. I wouldn't have somebody that age play it as of now. Even if it is historical, I'd say wait until one is a bit more mature. Even then, violence shouldn't be the kind of game anybody enjoys.
Any ways, my two cents.
-Seth