I have to go ask my wife, but I think when we used them in October '08 the minimum age was 12 to drive one at Fort Wilderness. It may have been because our family was renting them all together from Fort Wilderness, so Mom and Dad were out on the water with our kids. We did the hour rental, it was around $60. Look for specials and coupons -- we had coupons for a "free" extra 30 minutes with a one hour rental.
They are a blast to use -- they go a LOT faster than I thought they would!! Certainly try renting them at least once. The cost can add up quick (we had 4 boats for an hour -- $$$ ouch!!), but worth it to at least try them out. Having two people per boat is fun, and will save some bucks.