I have 1 year old twin boys who were out on our 27 foot sailboat starting at 5 months. Like other posters noted, they spent most of the time in their infant seats and though our sails were shorter than they used to be it generally worked out quite well.
We always go out with at least three adults (usually me my wife and a grandparent) so there is one person for each kid and one for the boat. I'm a bit more nervous about the coming sailing season because of the toddler factor, as noted by Faster. The kids don't sit still any more (ever!).