Why should there be any minimum age? Enough babies are "airborn" and it is just a natural wonder. Yes, if they are very little, you have to place a caring person direct next to it and you should avoid rapid climb and descent if you find out the little one has a problem with it, but then - no problem.
My youngest passenger was 35 minutes old and we flew the little one born a bit too early out of the Canadian wilderness to the next doctor to let her check ok. Not really a problem, even with mountain flying and rapid descent for this one.