From the train line site:
“The 16-25 Railcard – sometimes called the Student Railcard, or Young Persons Railcard – is a great way to keep travel costs down when you’re on a budget because, with one of these in your hand, you’ll get 1/3 off your train tickets.
You can use this Railcard to get cheap train tickets for your journeys to work, university or college, as well as using it for any journeys you make in your own free time. You can even use it to get cheap train tickets when you travel First Class, making long journeys more comfortable. There are no travel time restrictions with a 16-25 Railcard, so whether you have an early morning lecture, or you’re rolling home in the wee hours after a night out, you can still apply your discount.
The only requirement? As the name suggests, you have to be aged 16-25 when you buy the Railcard to qualify for the discount. And it doesn’t matter if you look young, sorry, you’ll have to provide ID to make sure you’re within the age limits. You can either buy a one-year Railcard for £30, or a three-year Railcard for £70, which can be bought up until the day before your 24th birthday.
Cheap train tickets are just the beginning with a 16-25 Railcard, there’s also a 20% discount on Virgin Experience Days and up to 60% off selected London West End musicals.“