Beyond which one's too old to mature (3,5)
I believe the answer is:
age limit
'beyond which one's too' is the definition.
I can't tell whether this defines the answer.
'old to mature' is the wordplay.
'old' becomes 'limit' (I can't explain this - if you can you should give a lot more credence to this answer).
'to' says to put letters next to each other (I've seen this in other clues).
'mature' becomes 'age' (synonyms).
'limit' after 'age' is 'AGE LIMIT'.
Can you help me to learn more?
(Other definitions for age limit that I've seen before include "Maturity restriction" , "Regulation based on year of birth that sets down when one can no longer do something" , "Bar to those too old or too young" , "I am legit (anag)" , "Regulation regarding length of time one has lived" .)