Minor issue (3)
(Other definitions for nit that I've seen before include "Louse; fool" , "Idiot, ass" , "Egg; fool" , "Young louse" , "Parasite's egg made of tin" .)
'minor issue?' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for age limit that I've seen before include "terminus a quo, say" , "Bar to those too old or too young" , "Regulation based on year of birth that sets down when one can no longer do something" , "I am legit (anag)" , "Maturity restriction" .)