Who old is old enough to let my puppy swim?

My puppy is 7 and a half weeks old and he is a Yorkie and I wanted to know when should I let him swim? I have been getting him ready to swim like filling up the sink with an inch of water and letting him play in it and he seams to like it but I just wanna know when I should switch from the sink of a little bit of water to the kiddy pool with more water. I want to make sure I don't traumatized him if I put him in the pool to early.