At what age does a cat stop growing?

I'm trying to get an idea of how old Swanie is. The people at the shelter guessed at age 1 - 2 and told us he wouldn't grow anymore. Well, he grew some more. A lot more. He also exhibited behavior I associated as more kitten like, but he's settled down a bit from that now. The vet said there was no real way to tell except to look at their teeth.

Depends on the cat's build, really hard to determine exactly with mix breeds.

In large breed cats some take up to 2 years to mature, in average size breeds, 12-18 months.

He might have been closer to 1 then. When we took in Whitey he was 11 months and looked like a full grown cat. The sick thing is that he has grown even more!
I hope he is done growing because if he grows anymore I'll be calling him Rover!