You can't change the age date in Cashmanager - something to do with taxable value.
Every year I have to phone the helpdesk for the same thing. We sell dry hoggets in June and I code them as hoggets which throws my stock rec. out. You should code them to lambs and then from July on they're classed as hoggets.
Fingers crossed I remember this year lol :)
Maybe you could have a chat with your accountant?