Neil without going into my own dog history of breaking dogs into game of all sorts I believe that @ 18 months one is behind by a long time as imo those first months are the times when to introduce the pup to portions of what it will encounter in its life as a working dog.
Things like water whether flowing creek or still dam,dead animals,live animals,distractions,noises both constant and sudden,gunfire,people/strangers,riding in and on vehicles,other dogs whether mellow or aggressive and the list go`s on and on all these besides mandatory sit stay shut up etc!