If your dog is under 10 years old then you might be able to find pet insurance coverage that will cover the older dog.
But usually once the dog gets to 9 or 10 years old it can be difficult to find an pet insurance company to sell you a pet insurance policy for the older dog.
Larger dogs that are 9 to 10 years old are getting into the old age part of their lives and will usually be dying anyway within a few more years so even then it may not even be worth it to get pet insurance if your dog is an older dog as in 9 years old or above.
Also pet insurance will not cover any pre existing conditions that the dog had prior to getting the pet insurance.