I suspect it'll be much the same as the existing 16-25 Railcard, namely you'll be able to buy one up until the day before your 30th birthday (week before your 28th birthday if they do a three year version) and it'll be valid until two days before your 31st birthday.
I suspect it may be generic, given Green is traditionally the colour for the Disabled Persons Railcard.
Surely it is more likely you can buy a railcard until the day before you turn 31, or buy a three year one the day before you are 29, and thus be able to have one until the day before you turn 32?