Pets won't age up

So I can't post anything on the sims forums about this for some reason so I will ask it here

I've been having problems aging up pets! I want to make my cat a ghost in game, but she won't die and any cheats I have found don't work...

So then I thought, there is an aging up pet treat! So I bought one and tried to use it on my cat, who is an elder. But there was no option to give her the treat, I assumed it was because she was an elder so I tried it on one of my adult cats and there was no option to age them up there either... am I missing something?