When did my 5 year old son become a teenager?

Last night I was telling son to get ready for bed. He takes a loooong time to get ready for bed. I usually have to threaten and give him a countdown, etc. So, last night I was telling him to brush his teeth, get his jammies instead of lying on the floor naked after his bath. He said to me "whatever". Whatever! When did he learn that word and how to use it properly in a disrespectful, dismissing way. He said it just like a teenager would.
Husband wasn't surprised that he has picked up attitudes from daycare. But I was. Of course, after his time out, he couldn't even remember what he had said. So I guess there still is some innocence left in him. It's a good thing we still have a few years before he actually is a teenager. I feel I am barely surviving this stage.