Insuring a senior dog is usually possible but structurally different from insuring a puppy. Premiums are priced on age, so the starting number is higher and typically continues rising at renewals.
Some insurers cap enrollment age for their full accident-and-illness products - while others enroll at any age - so the first filter is simply which companies will quote your dog at all. And the pre-existing exclusion does its heaviest work here: a decade of vet records is a decade of potential exclusions, because anything that already showed signs is off the table at essentially every insurer.