I took my daughter home on a 10 hour flight to China when she was almost 4 weeks old (we were in business class, granted) and my son Hong Kong to Beijing when he was 6 days old. There is no lower age limit with the majority of airlines but there will likely be a problem getting a passport in under 4/5 weeks for a newborn (DH was foreign office so pulled some strings for us). The ears are as mature at 1 day old as they are at 100 years old, nothing widens or gets bigger etc to make air travel easier. Apparently if the mum and dad suffer with earache when flying the baby is likely to as well, no matter the age of the child.
Check with your airline first and check passport issues. Make sure your baby is sucking during landing and take off (if only possible to do one then do take off).
I have taken my two on thousands of miles worth of air travel between newborn and 2 and never encountered a problem (though the other passengers may not have liked it!!!)