Minimum age for our yacht club for kids lessons is 5, they use Optimists and Fleas and are carefully supervised by two crews in zodiacs.
PFD's are an absolute requirement, though some of the younger kids use these really cool dinghy suits that are basically a short sleeved wetsuit with the PFD inbuilt into the chest and back. They seem to work great, cannot be taken off easily and are one less thing that can be lost or left behind.
We plan to start our son in his sailing program at the earliest age possible, he can already swim at the age of 6months and continuing swimming lessons and followed by water survival lessons are definately on the agenda.
Especially if he ever wants to set foot in a canoe or kayak (A far more dangerous vessel then even a small sailboat)