My 14 year old needs a new bed.
My 17 year old has a small double bed, I think you call it a queen size or 3/4 bed. It is one of those wooden framed ones which I've actually had for 30 years (obviously several new mattresses in the meantime!).
If I buy my 14 year old the same size bed I will have to get rid of all my single bed bedding.
When the 17 year old leaves home, the 14 year old can have the bigger bed that's fine, and we'll put the single bed in the 17 year old's room.
That's reasonable isn't it?