Originally Posted by
Often1
Depends on the age of the child.
For the child to be considered to be accompanied, he must be seated in the same cabin as the responsible adult. Otherwise, he must be 5-12 years old and be booked and paid for as a UM. at 12+, he's not a child and may sit alone.
Do bear in mind that JL is not going to put up with a significant amount of movement between cabins.
Different airlines have different rules
e.g. MH, for example, is ok if the accompanying person is on the same aeroplane , whilst Qantas requires them to be in the same cabin
Cannot assume what a particular airline will require and should check explicitly