The most common and most likely answer to this crossword clue is the 6 plus 4 letter word ANNUAL RING
I've seen this crossword clue in The Times
A
N
N
U
A
L
R
I
N
G
Buddy explains!
The straight part
We know that "a sign of age" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a sign of age = annual ring.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.