Our son took an Optimist sailing class last summer when he was eight years old. I believe eight was the earliest age allowed. We also believe that there was an expectation or requirement that he could swim unassisted for 50 yards, which he was capable of doing.
Here's a link to the sailing club where he took the class. If you look at the registration materials for the youth classes, the consent form notes the swimming requirment for the adult signing the document, but it doesn't necessarily note the requirement for the student...
My son enjoyed the class, but I think eight was the right age for him to take the course. Also, we made a big deal of him taking swimming classes before it and "passing the requirement for swimming" in the pool before he took the sailing class.