Hi Copyright, thank you for your explanation. However, I'm afraid that I can not understand what "who never showers" refers to here. Is it important for Bill or Ken to take a shower?"Ken is as old as Bill, who cannot drive a car." Ken is as old as Bill. Bill cannot drive a car.
"Ken is as old as Bill who cannot drive a car." Ken is as old as the Bill who cannot drive a car, but he's younger than the Bill who never showers.
The sentence isn't necessarily wrong. It depends on the context. Essentially if you use the comma, it means that the information that follows it is not necessary for the sentence to make sense. If you don't use the comma, then the information is important.Essentially, the second sentence is wrong; the comma is needed. Without the comma, it would sound as if you were talking about two different men named Bill, one who can't drive a car and one who can. Both Copyright and I are pretty sure that's not what you intended.
By the way, cannot is one word, not two—although we normally use the shorter version, can't.