[dragonsky67]

I'm obviously missing something here. 18yrs old = Adult in most parts of the world.

Is the problem that they are playing games, or is the problem that they are not supporting themselves and spending their time in an activity that you consider to be time wasting?

At some point people become responsible for their own actions, this depends upon the person, but in most cases 18 is the age where they make decisions about sex, alcohol consumption (in most of the world) and most other activities participated in by adults.

If this person (not child) is doing something you don't like under your roof then it's your responsibility as others have suggested, to talk to them like an adult and explain your concerns rather than start a war of attrition that you will almost certainly lose.