I booked a number of award tickets to LHR, one of which included a child under 12. At booking, the UA's website quoted me an amount in taxes that was about $109 less than the amount UA tried to ultimately charge me. I realized the discrepancy because more than 12 hours had passed without ticketing of the reservation. I called reservations, who saw that my credit card had been authorized for the lesser, original amount, but the system had tried to charge me for the $109 additional amount, which resulted in suspension of the ticket. After involving a supervisor, UA's response was they had no choice but to charge me the additional taxes; they would have given me the lower amount had it been a fare misquote. They couldn't explain the reason for the difference.

I did some research and determined that the website was charging the correct amount of taxes at booking because I properly identified the minor passenger's age at booking. But at ticketing, UA's system somehow ignored the child's age designation and made everyone adults for purposes of computing taxes.

This functionality affected the tax amount because beginning in May 2015 the UK will no longer collect the Air Passenger Duty for children under 12 in economy or E+. (In May of 2016, the APD will be abolished for children under 16 in the same classes of service.) The $109 (or £71) difference in taxes ended being the ADP charged on the child. After explaining this to UA, they did refund me the difference, and said they will alert website support.

Never thought it was important to indicate the age of the child when travelling with a minor. I rarely do it, but it can be helpful in some instances.