Railcards in the UK have a complicated history, but essentially there's a sliding scale of discount you can get up to the age of 30, in order to encourage more young people to use the rail network. You get more discount under 18, some between 18 and 25 for one price, and some between 25 and 30 for another price.

Additionally, the way the discounts work is that you buy a pass that lasts for a period of time. That then needs to work with your current age. i.e. you can't buy 2 years of discount for under 30s at 29, you can only buy 1 year, so they need to share your age and I think possibly in some cases, date of birth.