My oldest daughter went on our Hobie 16 (in the Gulf) when she was 1 year old and never looked back. 4 Years later daughter no. 2 started going out on our Stiletto 23 at the same age, same results. One thing we stressed though was that they didn't get on the boat without wearing a life jacket, never, ever. That is the way they learned from day one and it was always automatic, never an issue. Didn't even a have to ask. Now my daughters are 19 and 23 and still love it. Be careful, they start bringing along friends. Have plenty of food and drink!
Make it fun!