It is really not a boarding time. A more accurate term would be something like "scheduled terminal arrival time." If you arrive at the terminal before your scheduled time via any means other than
DCL transportation, whether you are allowed right in or kept waiting a short while will depend on how full the terminal is. The big ships have enough passengers to exceed the number the fire marshal allows into the terminal at one time.
After boarding starts, people rapidly move from terminal to ship--no more crowding, no more waiting. While on most of our cruises boarding started at 11:45, on our May Fantasy cruise it started before 11:30.
After you enter the terminal and check in, you will get a boarding card with a number on it. This number has been determined by the computer at the time you did your on line check in and will not be changed by your actual arrival time--if you arrive at 11, you'll still get the pre-assigned number. You will board the ship when this number is announced (with about 100 or so of your closest friends). Boarding numbers go fast. Somewhere around 12:30 or 1, they will announce "open boarding" meaning that there are no crowds remaining and each family can board as soon as they have checked in.
SO...back to original question. If you arrive before your scheduled time, you may or may not be allowed to check in.
If using DCL transportation, you may enter as soon as the bus gets you there; if coming from MCO via DCL bus, you will have already checked in at MCO.