Perpetual advice not to show one’s years so much! (7)
I believe the answer is:
ageless
'perpetual' is the definition.
(thesaurus)
'years so much' is the wordplay.
'years' becomes 'age' ('age' can be a synonym of 'years').
'so much' becomes 'less' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'age'+'less'='AGELESS'
'advice not to show one's' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for ageless that I've seen before include "Forever young" , "Staying young" , "Eternal - like Peter Pan" , "Small amphibian" , "Not growing older" .)