Vet tech here … most of our older pets don’t die of old age, they die of cancer. It sucks.
There’s no way to prevent cancer. It’s a mutation of cells. However, to keep your good boi or gurl with you longer some things that really make a difference: a quality diet (please don’t feed them the cheapest stuff, it’s like humans eating the cheapest foods; not terribly healthy though it may fill all the nutritional requirements), all the regular vaccines (for the love of Dog, make sure you get your puppies the Parvo vaccine!), and a good flea/tick and heart worm preventative. Exercise and not letting them become overweight, and regular dentals are even better!
Also, screw you if you buy your vaccines for your domestic pets at the feed store or the tractor supply (talking dogs and cats here, not large farm animals). Just because it’s cheaper and has the same chemical name doesn’t mean you’re giving the right dose, amount, at the proper location, or getting the assurance that those vaccines have been treated and stored in the correct manner. You get what you pay for.