If you're asking this as a parent, don't let your kid(s) play it... just don't, get them outside socialising with their friend in person. It will keep them active, and it will save you from having to get them off the computer.
Minimum age is 13. Good game when you've understood the deeper (non-obvious) mechanics available towards the max level. Plenty of great quests too, in fact that's the best point, it's an RPG with SP-esque quests contra fetch and kill quests.
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