Because of the extra risk of health issues for older cats, some insurers have an upper age limit and won’t offer new policies for cats above it - examples include eight and ten years old. Some insurers have no upper age limit, but as your cat ages there may be certain restrictions on what’s covered.
If you take out lifetime pet insurance when your cat is younger, and continue to pay premiums, the cover will extend throughout their life - no matter how long they live. Just bear in mind that premiums are likely to increase as your cat gets older.
