I am not too sure I would disagree with that - only though because my cat was a very small cat until she was about 3 and then she seemed to grow(!) She is 8 now and still on the smaller side of average but she definitely grew after 18 months.
My vet told me that usually they stop growing by the age of three, but some types can continue to grow until around four years old. Mine is part maine-**** and was one of those who grew til he was four and so I enquired at the vets.