Is owning a dog at 20 a stupid decision?

It's all up to you. After the dog is an adult and trained, you could still have a social life. It's a good decision at 20 if you are willing to care for it and play with it plenty. If you are going to be out for 10 or more hours socializing, its not recommended unless you can drop it off at a friend's house or a dog camp for the day.