Most airlines would allow any infant that is about 48 hours old to travel. But they also advised you to check with the doc frist.
From British airline site.
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Fare discounts and booking
We offer discounts for infants and children as follows:
Infants
Infants are those under two years old at the time of travel.
The youngest age an infant can be to travel is 48 hours old. We also advise you to ensure that the treating doctor agrees to the infant travelling.
An infant fare is charged at 10% of the adult fare when the infant does not have his/her own seat, but sits on an adults lap.
If you want your infant to sit in his/her own seat (they must be between 6 and 23 months), you will be charged the child fare. This cannot be booked online. Please contact your local British Airways office if you wish your infant to have their own seat. You will need to ensure that you have a suitable car seat or have requested one from us for your child to travel in.
There must be one adult accompanying each infant seated on an adults lap.
- if one adult wishes to travel with a second infant, this is only possible if the second infant is aged between 6 and 23 months, he/she must be seated in an approved car-type seat and we will charge the child fare
- if the second infant is under 6 months you must travel with a second accompanying adult or a British Airways escort for which there is a charge