Is it bad for a single 20 year old adult to play video games

No, why would it be? It's a relaxing hobby like many other things. Others play golf, rides horses, collect stamps, watch TV, listen to music, read books etc. Nothing wrong with it as long as it doesn't take over your life. When kids grow up to becomes adults they usually keep the hobbies and like games they used to like. That doesn't just stop.