Do adults who play computer games...have low IQ levels?

Depends, what kind of games, how old, and how stupid were they to start. If someone sits in front of the screen 24/7 just playing, they will be stupid. You have to consistently exercise your brain just like other muscles.

Yeah. Have you read some of the questions in the video games category. It's like hell if you're an English teacher. There are so many grammatical errors and spelling mistakes. What's even sadder is that I suspect most of them have spellchecking on and since they've made so many mistakes before that the spellchecking assume that it's what they want to spell.

Some likely do but there is no correlation between one's enjoyment of video games, their age and their IQ.

No, games are just entertainment.