Not old enough to translate aged rune (8)
I believe the answer is:
underage
'not old enough' is the definition.
'translate aged rune' is the wordplay.
'translate' is an anagram indicator (translate can mean to transform).
'aged'+'rune'='agedrune'
'agedrune' with letters rearranged gives 'UNDERAGE'.
'to' acts as a link.
(Other definitions for underage that I've seen before include "Not old enough to exercise legal right" , "Need a rug (anag.)" , "one too young" , "Not legally old enough" , "Not yet eligible to vote - too young" .)