Re your query, best for you to check with the arilines.
However, as I have been travelling quite a bit, I could see babies as young as 1-2 month old were on board.
Personally, I feel that if they can refrain from the flight, it would be better coz' at such young tender age, they may be exposed to a lot of germs/virus on board. In this aspect, not adviseable to catch a flight at such young age.
Take care. See you around. Cheers.