There aren't any magic secrets (most is genes, second is decent care) but you can increase the odds by good health screening and keeping your dogs svelte and with good teeth and gums. Lots of the dogs I know that have lived to old age were fed supermarket foods, no supplements etc. Some I know that have died early were on all sorts of trendy supplements, raw diets, premium foods. I know anIrish cavalier that lived to just shy of 19 -- no special fuss, just supermarket foods. They wouldn't be my choice, as I think them quite mediocre, but neither would I (waste) spend money on the most expensive foods. Our family Pyrenean in the 60s and 70s lived to over 13 -- like reaching 18 for a cavalier -- and had nothing but cheap supermarket foods, table scraps and the occasional raw egg for her coat!
Annual cardiologist screening and if possible, an MRI so if a dog has a syrinx, you can give something like frusemide to inhibit or even stop or reverse development. If you see no symptoms and don't know your dog has a syrinx SM could slowly progress til at a serious level. The low cost screening available in the UK makes a basic MRI very accessible to a pet owner. MOST cavaliers will eventually have MVD and all research indicates well over half will likely end up with SM as well -- so minimise those risks. On hearts -- a cardiologist will pick up issues long before a vet. Some here are good on supplements for dogs as they get older -- that they believe are helpful for heart health. I don't think there's much done to verify many, if any, of these supplements helping in any way but some have anecdotal evidence. Pat for example has dogs that have reached a very good age despite some health issues.
She will know what studies are out there too, I've no doubt.
What shortens life in cavaliers is MVD, SM and as always, being overweight. Even a few pounds can cut *years* off a dog's life (an average of 4 years in a study on overweight -- but not obese -- labradors. Assume the same with cavaliers. Being overweight as with people increases risk of and lowers onset age for everything from diabetes to heart disease to cancer. Owners DIRECTLY shorten their dogs' lives by letting them get overweight. A dog will not get fat unless given too much food by owners, and not exercised adequately.
PS: A little reminder to all that using a subject heading that tells a little more about what your thread is specifically about would help people both find info from it that they might want or add to it. eg 'how to help our dogs live longer' as 'how to help' could be about anything from fundraising to housetraining... :lol:. Adding in some tags to a thread also helps people find info in future.
Many of us would look at a subject heading first in deciding whether to read something so putting an informative subject heading really helps (rather than 'advice needed', use 'advice needed on housetraining'; rather than 'why does she do this?, use 'why does my dog bark at night?').