A suggested age of 4 is the typical recommendation as children are old enough to understand the lessons from an instructor and learn how to swim effectively. If you are willing to spend the many hours required one on one with your child, then you can start teaching them water basics which will help their safety if they happen to end up in water unexpectedly at an earlier age.
In winter and especially in cold climate areas you will find that these pools are heated - otherwise the pool might be quite empty!
50 metres is the Olympic size of pool. Most outdoor pools at public will be of this size.
Yes, Swimming is a great form of exercise. It is useful also for people who may have back and joint problem where the buoyancy of the water can help take pressure off joints while exercising.