Somehow not new, yet not of a significant age (6-3)
I believe the answer is:
twenty-one
'not of a significant age' is the definition.
(21 was formerly the UK's legal age of majority)
'somehow not new yet' is the wordplay.
'somehow' indicates anagramming the letters.
'not'+'new'+'yet'='notnewyet'
'notnewyet' anagrammed gives 'TWENTY-ONE'.
(Other definitions for twenty-one that I've seen before include "XXI - used to be the coming of age year" , "Target total in blackjack or pontoon" , "Card game, blackjack or pontoon" .)