Swimming, as a competitive or recreational sport, has been around pretty much as long as mankind. The benefits of learning to swim are huge, from saving yourself or others from drowning, to keep yourself fit and healthy. The sport gives pretty much your whole body a workout, with a very minimal risk of injury and is a great way to socialize and cool down on a hot day.
The younger people learn to swim the better to gain the benefits mentioned above. Children can start learning to swim as soon as they are born. Many small children can actually swim before they walk, they are used to water having spent 9 months in liquid, inside their mothers. With small children though it is best just to play with them in the water first and let them become accustomed to idea of water and more importantly breathing above it. A great thing to try and teach kids of this age is to breath in above the water, then breath out underneath it, as this is what they need to learn to become proficient swimmers in the future.
At age 3 is probably the best time to put them in proper swimming classes. At this age they are aware enough to follow instructions and socialize with other kids in the group. As well as improving health and fitness, swimming can improve your emotional health. Being in water, in a safe environment can be a very relaxing experience. Kids enjoy swimming so much it is usually very difficult to get them back out of the pool.
There truly are only positive things to say about teaching your kids to swim, and from a very young age.