However, those 18 year olds cannot add their railcard to an adult Oyster card to receive a 1/3 discount as holders of a 16-25 railcard can. They must pay adult fares until their railcard expires and then buy a 16-25 railcard or buy a 16-25 railcard on top of their 16-17 one.
16-17 railcards no longer expire when the holder turns 18 instead they expire a year after purchase. The 16+ Oyster card expires in September in the year after the holders 18th birthday. Therefore, there are 18 year olds with 16-17 railcards but cannot have a 16+ Oyster card.