A hair product I’m regularly into — you’re never too old, unless it’s applied (3,5)
I believe the answer is:
age limit
'you're never too old unless it's applied' is the definition.
I can't judge whether this defines the answer.
'a hair product i'm regularly into' is the wordplay.
'hair product' becomes 'gel' (hair gel).
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
The alternating letters of 'into' are 'it'.
'a'+'gel'+'im'+'it'='AGE LIMIT'
Can you help me to learn more?
(Other definitions for age limit that I've seen before include "Bar to those too old or too young" , "Regulation based on year of birth that sets down when one can no longer do something" , "terminus a quo, say" , "I am legit (anag)" , "It can mean one is too young or too old" .)