I was 13. My parents had a ski boat on a nearby lake in Michigan. I got tired of waiting for my next turn at skiing so I "designed" and built a sailboat from a 4 x 6 piece of plywood with two styrofoam surfboards tied to the bottom. Mast and boom was 2 x 2 with piece of plastic drop cloth for a sail. Had centerboard and rudder from scrap lumber. Sailed all over the lake and could even go to windward (amazing now that I think back). Got lots of looks and some encouraging remarks.