Alison & Carol...it all depends on your comfort level about how close to departure time you get to the airport.
Most of the time the ship clears US customs and we can begin getting off early. One time we booked an 11:15 flight (it was either that or 5:30 pm) and even though there was a problem with a few people not reporting to customs so the ship could be cleared (we docked way before 7 am)...we finally did get off around 9 am or so and made it just fine with time to spare.
The airport in Ft. Lauderdale is less than 5 miles away from the cruise port and took us no more than 15 minutes to get there.
Olivia however does recommend 12 noon and beyond to play it safe...because problems do come up from time to time...and usually just when you need to get off early