Afaik, yes it can develop at any time with a cat who was exposed to and contracted corona cat virus, most cats do not get very ill with it and you would probably not know they had it unless you tested for antibodies- it’s not the same as covid, it’s a different virus which only cats get. About 75-80 % have been exposed at some point. FIP is a mutation of that virus and it doesn’t always happen immediately after being exposed to it. I don’t think they really know why.
There is a much better treatment now- I don’t know if it’s been approved by the FDA yet, but I think one of them was. But it’s still very expensive and some cats are surviving.
There are also different forms of FIP, including wet and dry. Wet is acute and is usually worse. Here is a link…it’s more common to happen in kittens around 5 months up to two years old however it can happen at any age. It is not contagious, the mutation can’t be transmitted to another cat from what I understand.