If the clown was wild caught there maybe no way to know how old he (she) is, since he could have been a nonsexed small fish for years. On the good side, I have read that clowns have been kept in aquariums up to 30 years, but my oldest is only about 4yrs old.
I lost a small male exactly they way you discribe it ....he just got bloated, a little, quit eating and died....my guess was some sort of intestinal block...he was a pig and would eat mysis untill he couldn't move
.....just my guess ....good luck