小孩要how old 要買機票???

3 個月以下不需買票
2歲以下半價
2歲以上 比75%
但有些特價機票, 是不設小童票, 即要比成人價.

Infant will be before age 2 birthday. It will be 10% of adult published fare (not after discount). However nil seat will be provide the infant. Only if weight below 10kg can be request baby bassinet

If over age 2 and before age 12 it will be Child fare. Price may be 75% of adult discount fare. However some-time the lowest fare will not offer Child discount and it will be same price as adult. It must be checking with your agent.

Some domestic sector like inside USA and inside Australia for some airlines are free for infant who together with a pay adult. (cannot be redeem ticket)

Besides Infant or Child age will be count for the whole trip. If departure a Infant but return day will be after age 2 birthday. Child fare will need to purchase. (airlines will said you can issue oneway infant and oneway child but after check fare, price will be more than issue a wholly child fare)
and so do the child if age between 12-13year. Itmay need to issue a Adult for return will be after age 12 birthday