At what age can a child learn to swim?

Babies are natural born swimmers! During the last baby fair at Singapore Expo, I signed up for a trial class for my son who will be 7 months by then. I signed up with Litttle Splashes Swim School for $45 per lesson. They have their own heated private pool which I found was important. Each lesson is about 30 minutes long and I planned to sign my 7 year old daughter as well. I haven't gone for the trial (in mid Feb) but I've heard rave reviews from other mummy friends who have attended.