How old does a child need to be to sit separately from parents when flying?

Any person above 2 years must have a seat, but where is that seat? That's something left for common sense and not covered by any policy I am aware of.

Seating policies usually go like "every effort must be made to ensure that families are seated next to each other" and that's it, and as we know, as long as that "effort" costs money, airlines might ignore it! Many airlines make extra money from allowing passengers to reserve seats. They will not implement policies that will add extra responsibilities to them and let them lose some good money.

However, out of personal experience, it will not be that hard to find a passenger who is willing to switch seats to let a mother sit next to her kid. Just give the cabin crew a hint about that and let humanity deal with the rest.

In booking another transatlantic flight with British Airways, there online system is more informative. It says

If you don't pay to choose your seats, British Airways will allocate seats to passengers travelling with children (aged 2-11 years) approximately three days before departure. We aim to seat families together; however where this is not possible children will at least be seated with one adult in the same booking.

Happens often enough that there appear to be no rules at all about seating children other than not in the exit rows. The airline seat-reservation model certainly has no exceptions for age, but if you call the reservation line they will probably accommodate you next time.

If not, maybe enough of:

Can I really sit a 4 year old next to innocent strangers and let them, and the flight attendants, deal with the resulting tantrum?

will convince the airlines to change things.

In the good old days when I was younger and travel was more .... civilized and less reliant on technology, family seating was a non issue. We were 6 kids and two adults and were all seated together. In fact, the row of seats that had baby bassinet hooks were exclusively reserved for families.

These days, the only rules that airlines absolutely must follow are:

1. Any person 2 years of age or older has to have separate seats.
2. Any person deemed unsuitable (due to age, language or other restrictions) cannot be seated in an exit row.

Other than that you are at the mercy of the airline and your fellow passengers. These days I find that it is often cheaper to rely on humanity than to pay for reserved seats.

Another interesting twist on this idea happens often in the Middle East (but I am sure elsewhere as well, since its a Muslim issue) where it is not allowed for women to be seated next to unrelated males.

Some families are very strict about this; others not so. I have seen this often where the poor cabin crew are left to play musical chairs trying to satisfy everyone's needs. This really becomes an issue on full flights (usually during the busy summer season).

So, what can you do to ensure a safe flight for you and your children?

1. Book as early as possible, to get preferred seats.
2. Pay for seat selection.
3. Ask the crew, but ask them once everyone has boarded - try not to create a bottleneck when there is a large queue of people standing in the isle.
4. Ask the person sitting next to your child. You will find that most people are very understanding. Offer your seat for theirs before asking your neighbor to move.

In the USA, starting this month, there's new legislation on family seating. Inclusion of the language on family seating in the FAA reauthorization bill comes after similar legislation last year called the Families Flying Together Act of 2015 was introduced. The legislation directs the U.S. Transportation Secretary to look at establishing a policy directing airlines to allow children age 13 or under to sit next to an accompanying family member who is older — at no additional cost. In addition, the TSA can no longer separate parents from their children during screenings.