The most common and most likely answer to this crossword clue is the 12 letter word CARBON-DATED
I've seen this crossword clue in The Guardian
C
A
R
B
O
N
-
D
A
T
E
D
Buddy explains!
The straight part
We know that "Age thus settled" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case age thus settled = carbon-dated.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.