Aibu to resent paying more for DD’s train ticket now she’s turned 16

[Kmetsch3]

DD is 16
She’s still in school and will be for two more years
How can train companies, and others, justify charging her ‘adult prices’?
I haven’t noticed her income going up

I resent paying adult prices for my eldest DD who will be 12 in a couple of months. I don’t know at what point a 12 year old is classed as an adult 😡