From the Boston Marathon website:
"The qualifying times below are based upon each athlete's age on the date of the Boston Marathon in which they are participating."
So, if you run a 3:30 at age 44 in the qualifying window, but subsequently turn 45 before the Boston Marathon, your time would be a qualifying time in the 45-49 AG (which has a 3:30 standard).
But don't take my word for it. I haven't run it yet. Go to the source:
http://www.bostonmarathon.com/BostonMarathon/Qualifying.asp