i don't think that there is an earliest, you just have to bear in mind that you may be feeling pretty crap for the first 12 weeks, and 15 hours is a long time to spend in an aircraft toilet! on the other hand, you may feel perfectly fine. As regards a latest date, I believe that this can vary from airline to airline, but it is the pilots decision whether to take you or not. Also, most of the deadlines given are to fly without a doctor's letter, you can fly later as long as you have a letter from your doctor, and the pilot is willing to take you - however, unless you have any major problems, I think that even if you got pregnant now, you would probably still be within the limit of most airlines.
I had a boss who was still flying at 38 weeks, but that was within the UK and she never actually told them how many weeks she was, she just had a small bump and walked onto the flight and was never asked how long she had to go!