Although I have had a couple of negative experiences with Petshealth, I must say that they seem to be the ONLY pet insurance company willing to insure older pets. They will insure dogs up to 13 years and cats up to 15 years. They will even insure the pet with their top plan, regardless of age, if there are no pre-existing conditions. I have a 13 year-old cat who has never been sick a day in his life. I was able to insure him with Petshealth under their most extensive plan.
A few things to watch out for, though. First, unless you get their top plan that covers chronic and/or recurring illnesses, if your pet is diagnosed with some illness and/or injury in one plan period, as soon as the plan period renews, it is considered a pre-existing condition and coverage will stop. This is a bad deal if, say, your pet is diagnosed with cancer a week before your plan renews because in just one week, the coverage for the cancer will stop. That's crappy. Also beware that any congenital and/or hereditary condition is not covered on ANY of their plans. So, that immediately excludes a boatload of illnesses as there are lots and lots of disorders known in hundreds of breeds.
But, again, if you need to insure a senior pet, this is probably the only option you really have.