When one is so young, there's the difference (5)
I believe the answer is:
three
'when one is so young' is the definition.
I can't tell whether this definition defines the answer.
'there's the difference' is the wordplay.
'the difference' indicates an anagram.
'there' anagrammed gives 'THREE'.
Can you help me to learn more?
(Other definitions for three that I've seen before include "Subject of 1 9" , "A crowd, you might say" , "Start to count back, briefly" , "Feet in 3 Down" , "Number of feet in a yard" .)