They board as early as 11:30 - but officially I don't think its until sometime after noon - maybe even 1:00.
Guests are required to be on board 1 1/2 hours before sailing - sail is at 5:00, so that means 3:30. People HAVE boarded later - often those using Disney transportation getting in late to the airport. But don't count on showing up at 4:45 and strolling on the ship.