Hi.
I’m sure you probably all know this but to save you the trouble of looking everything up, Heathrow Express no longer sell 5p first class fares for 15yr olds and under.
I believe they never meant to offer them in the first place, sadly.
Welcome to the forum ... TT-ONR-NRN (Trying to decode your user name there - looks like a poster code?)
Even with the simplest of networks - HEX being just Paddington to Heathrow - it's easy to get fare anomalies or gremlins in the system.
Could a 44 year old have bought three 15 year old tickets for a total of 15p and added them together?