That's not an easy question where Hawaiian Cypraea are concerned.
 
I have seen what appear to be 'juvenile' cowries setting on clutches
of eggs. In one case, I observed a bulla stage Cypraea isabella on
eggs.
 
If they can reproduce, are they 'adult' yet?
 
On the other hand, I have established that Snakeshead Cowries (in
Hawaii) reach a 'terminal' status and continue living for at least
a year afterwards. Is that the 'adult' stage?
 
 
Aloha,
 
makuabob (a.k.a. Bob Dayle)