My son will be 21 month old on departure but when we return he will be 2yrs 2 months ,How do the they work out the cost of flight ? ,Will he be classed as 2 yr old for the all flight costs.

Ask the airline

Then you have recourse if it hits the fan. Send email so you have the answer in writing

In here the advise might be right or wrong

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In general it is based on the age at the time of departure of the first flight.

EG:

12 years and over is classified as an adult.

Birthday: 30 June.

Current age 11 years.

The child should still have a child's ticket if the first flight (say Brisbane - Bangkok) is done before June 29. If the departure date is June 30 or after, it will be an adult fare.

This works if it is a return ticket (BNE-BKK-BNE) purchased. If you by a single each way then it will be based on the age at time of flight.

As far as I'm aware, this rule hasn't been changed - but DO check with the airline or a halfway decent travel agent.

The OP's case may be slightly different if s/he is nursing the child, then wants a seat for the return.

If it's a long flight, having a nearly 2 year old on your lap might be quite a trial. I would probably get him a seat both ways.