I don't think there is an age limit to trade stuff in, but i think it'll be a good thing to have someone older do it so you can't get played out of some money

I would have thought it would be the age that the game is

Sell your stuff online , doubles your profit

When ever i tradded stuff in when i was younger they would only allow to trade then in for exchange unless i was with someone over 18 which they would then do for cash

You have to be the age or above of the rating in order to trade it in.
The policy is slightly stupid but makes sense since you can't buy if you're underage (though I think ages should just be guidelines anyway as most people don't follow them and the parent should make their own decision on how mature the child is, but that's another discussion)

You have to be 18 and have ID proof in the UK for 18 rated games.

Dont trade in at GAME you'll get **** all for them.

It depends on what you're trying to sell. e.g to sell GTA you would have to be 16. I tried to sell COD Ghosts but I couldn't as I was only 15 but I was able to sell Forza 5 as it's a age rating at 3. Hope this helped

You have to be 18 to get cash, i believe you can get in game credit at 16.

RedWood wrote DONT TRADE IT IN AT GAME! SELL IT ON EBAY!!

Noooo the secret of saving money has been revealed. D:

But never sell to gamestop, or any other "retailer" as they rip you off as much as pawn shops do.

Actually gamestop is technically a pawnshop.

ALWAYS sell online or locally. If you're gonna accept less than $250, you sir, have been ripped off.