This is most certainly the best Mass Effect game i have played. The story is great, the combat and gameplay is incredible, and the overall experience of the game is probably the best i have ever played. Throughout the game you will step back, look at the clock, and think to yourself: have i really been playing for this long? I highly recommend this game to any kid whose parents will allow them to buy it.
This is a great game, but there are some things you need to be aware of. The first, and most obvious thing is the violence. This game requires you and your allies to shoot and kill people, aliens, and robots. However, although most of the gameplay is centered around combat, the violence is neither excessively gorey nor uncalled for. The premise of the game is that there is a race of sentient machines known as The Reapers whose goal is to "harvest" all life in the galaxy. All combat is based around the concept of saving humanity, and all the other races of the galaxy.
There is an option to romance a crew member (the character you play serves as commander on the spaceship Normandy SR-2). However, although your character can have sex with the person you chose to romance, there is no nudity (aside from bare chests for men and a bra for women), and there is no way that you can be unfaithful to the person you chose. Men can romance other men and women can romance other women. It is possible to go through the entire game without romancing anyone.
There is strong language throughout the game, but nothing a teenager couldn't deal with. F*** and S*** are used, as well as D**n and G*dd**n (more frequent), but swears are not used that often.
The whole game requires you to make decisions that effect the galaxy around you. Your choices will also have a positive or negative effect on your crew as well as yourself (quite literally, the more of a "renegade" you are, the more scarred and disformed your face becomes), rewarding players who make good decisions and hindering players who make bad ones, turning the player him/herself into a positive role model.
SPOILER ALERT (kids stop reading here if you are still reading):
The ending of the game requires the player to make a choice, either destroy the Reapers, control them, or fuse all humans and synthetics together so that they can coexist (only available at certain military strengths). This was one of the most fulfilling endings to any game I have ever played, and i highly recommend that you allow your kid (if s/he is older than the age of 14) to play this game.