According to ESRB, this game contains: Blood, Language, Suggestive Themes, Violence
Parents would be well-advised to keep children away from the game. It's not that any of the ESRB's warning nouns are really all that warranted, but more that the game will be abominably hard for younger players. However, there is a significant level of violent gunplay in the game, and although I wouldn't say it was very gory, it's pretty clear that enemy soldiers are getting drilled full of holes by automatic weapons. I'm not clear where the "suggestive themes" were unless you count the faux porno magazines that Snake can drop to distract guards, and I don't. The story is written at an adult level so the tone is very serious and intricate at times, and there is a small sprinkling of very mild swear words, almost unnoticeably present. Still, despite all that, I'd say the game would be safe for most older kids if they can cope with the game's difficulty.
Metal Gear fans should know what they're getting into. The story, tone, and characters are all in line with the rest of the franchise, but the gameplay is radically different. Those willing to learn the ins and outs of a hybrid strategy system will ultimately find a worthwhile game, but those wanting fast-paced espionage action should look elsewhere.
Strategy RPG fans will likely get more out of Ac!d than the average Metal Gear player, but they should be prepared to learn a system that initially looks like others in the genre, but has enough odd quirks to throw them for a loop. Once the mechanics are mastered, the second half of the game is a very fine experience.
Deaf and Hard of Hearing gamers will have no problems since all communication is delivered through on-screen text. There are no auditory cues that are not co-represented visually through alarm meters, or other such devices. I'm very happy to report that Ac!d offers full access.