Being of tender years, they, fun souls potentially (12)
I believe the answer is:
youthfulness
'being of tender years' is the definition.
(being youthful)
'they fun souls potentially' is the wordplay.
'potentially' indicates an anagram (letters have the potential to spell a new word).
'they'+'fun'+'souls'='theyfunsouls'
'theyfunsouls' anagrammed gives 'YOUTHFULNESS'.
(Other definitions for youthfulness that I've seen before include "Freshness and vitality" , "Fresh, vigorous condition" .)