Fish continue to grow through out their lives albeit very slowly once they reach a certin size, so an 18" O would be pretty old I would imagin. I think 18" is about the upper size limit but who knows what lurks out there. I mentioned a couple of months ago about putting a fish with the potential of reaching 18' into a tank I thought was too small and the OP tried to jump all over me about over exagerating the max size of an Oscar.