The minimum suggested age for children to fly is two- to three-months. Waiting until this time will allow a baby's immune system to become more mature which will mean a world of difference with respect to air travel. If you can't wait that long you'll want to check with your physician and you'll probably hear that you can travel with a baby as young as two weeks provided that the baby has a clean bill of health and that there is due cause. Also note: I've heard that most airlines will not let a baby that is less that a week old to fly without a doctor's note...though I don't know that they'll really be able to tell the difference between 6 days (less than a week) and 8 days (more than a week). It's probably a pretty loosely enforced standard.
Our doctor was super clear (whether he was right or wrong is of course debateable); don't fly with the baby before 3 months old and if you do, try to stretch out the time between the outbound and return flights (to minimize immediate repeated exposure to recycled air).