That depends on the airline. Each airline has its own rules regarding "UMs" (unaccompanied minors) depending on the number of stops, length of flights, etc. Some airlines will allow children to travel unaccompanied as young as age 5, particularly if it's a non-stop flight. Check with the different airlines that fly the routes you're interested in for their rules and, of course, expect to pay a surcharge for this service.
However, if you're talking about newborn infants, I believe most airlines leave that up to the discretion of the parents and their doctor.
There is no regulatory minimum age in the US, but it is possible that some airlines could have their own minimum age rules. In general, it is presumed by the airlines that all passengers are healthy enough to travel, and those that are questionable, should have a Doctor's permission to travel. If you have questions about a specific airline, you should contact them directly.
Any child to travel alone with the guidance of the airline is called an "UNACCOMPANIED MINOR". Lufthansa is one of the best airlines when it comes to the UM service. The minimum age to travel alone is 5 years and the maximum age is 18 years. Normally a fee is charged for the service. The UM is taken care of at from the check-in and handed over to the crew who then hands over to special handling department on arrival an they hand over to the guardians/ parents on arrival. If the child is in transit then they are again handed over to the next flight crew and so on.
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