Does not matter if you ID for a game or not...most the time the kid drags mom or dad in there and you ask the parent if they are ok with this M rated game and explain the content..the parent still says ok and forks over the ID.
ESP the Mexican kids with non english speaking parents(no not being racist here when I used to to work at that shitty corp gamestop this happened all the time) you would explain and the kid 'translated' to his parent..so they whipped out the card and you still have to sell it to them.
..even if the policy the person with the ID is the one who MUST pay for the game to try enforce this further didn't work.
Then there is the send big brother or sister in to buy it then big bro or sis hands the game over to little bro or sis as soon as they exit the store.