Although a baby can fly at a week old, many people suggest waiting till they are three months old.

This gives the immune system a chance to develop, to help combat bugs and germs through the recirculated air system of 400 or so passengers. Have also heard, though I can't find the reference, that the soft spot, ear drums and lungs are better suited to coping with the pressure, once they have had a decent chance to get used to working on their own. It may be hard, but my advise would be wait until the baby is 3 months old.

http://life.familyeducation.com/baby/safety/42027.html

http://www.babycentre.co.uk/baby/travel/flybabyexpert/