The most common and most likely answer to this crossword clue is the 8 letter word TEETHING
I've seen this crossword clue in The Times
T
E
E
T
H
I
N
G
Buddy explains!

The straight part
We know that "pain when young" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case pain when young = teething.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does TEETHING mean?
TEETHING verb- To grow teeth.
- To bite on something to relieve discomfort caused by growing teeth.
- The eruption, through the gums, of the milk teeth; dentition.