An 18-year-old is an adult. Parents have no right to enforce computer time restrictions on their adult children and should not try to.
If my kid is 18 and living at home, I have some say in the life. If they want 0 say, they can find their own way. Obviously, I don't give a rat's ass that my 18 year old games becasue as long as they have a healthy lifestyle, I'm glad they're enjoying themselves but if they're starting to become a hermit and losing friends, not working, not taking care of their health, etc, then they're going to have some pressure from me.
You don't get to live off the teet and also not listen at all.
I understand your comment and I just want to say that my comment is assuming we're past that point that you're talking about. I didn't want to write an article. Some kids can be stubborn. Anecdote: My great grandma tried that with my great uncle (all carrot) and he lived in her basement playing videos games until he died of a heart attack. I thought he had the best life because he got to be lazy, but it didn't server him very well. The end. Being a pushover and being considerate is a very fine line that is easy to fuck up.
If you want to go down the "no right" path...
Assuming the adult(child) is not contributing financially to living expenses, the child has no right to an internet connection, electricity, room & board, usage of any equipment that they do not own, etc. So while the parent has no right to tell their adult child what to do with their time, the adult child has no right to free resources.
If said 18 year old has the money to live on their own and play video games, all the more power to ‘em!
But seriously, the computer isn’t the issue. Kid needs to start pulling their own weight or they’re going to be in for a rude awakening.
Certainly you have the right to expect some participation in the household, chores, standards of cleanliness, safety, noise etc, just as you would any other person living under your roof (boarder etc).
You no longer have the right to define their schedule, gender, religion or personal preference. In short they have the right to make their own mistakes, you can try to guide, but you can no longer enforce, beyond inviting them to leave your home. I would suggest that you may want to compromise and keep your ability to influence and guide, rather than giving them the boot and forcing them into the world.
Letting a child, adult or otherwise, play video games 8 hours a day, is not going to have a positive outcome for the child. I have great sympathy. We were very permissive parents and our kids were not in a good space, mid-teens. We now have much better relationships with our kids, and they with each other. Fortunately we could afford counseling, coaching and group coaching. Establishing boundaries and consequences was step one.
do unto others as others have done to you?