The old "one-two"? Pretty young, actually (6)
I believe the answer is:
twelve
'the old' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for twelve that I've seen before include "XII" , "houses in a year?" , "Six twos" , "Last hour" , "Lunchtime" .)