I don't have one that's 15 I was just wondering what's the best stuff to help them live that long?
I don't try to keep the that long, it is not part of my goals. But I'll give my opinions.
Try to find someone who breeds for longevity. Hatchery birds are not bred for longevity. Many breeders don't. If you can find stock that has been bred for longevity you are way ahead of the game.
Do not get specialized chickens. The hybrid broilers, whether Cornish X or Ranger types, are bred to put on weight quickly and get so big they typically have health problems. The hybrid egg layers are specially bred to lay a lot of large eggs in a short time span of a couple of years, then they will be replaced. Because they are laying a lot of large eggs for their body size they are susceptible to laying problems.
I believe in strengthening their immune system. Expose them to the environment they will be living in early on instead of trying to raise them in a sterile environment. I don't use medicines as a preventative, I think that weakens their immune system and helps breed stronger pathogens. If I see a need to treat I'll treat, but I don't do that to make myself feel better. I have to have a reason.
Keep their living conditions dry. A wet coop or run is a dangerous coop or run.
Keep water clean. Pathogens can breed in dirty water.
Keep a closed flock. Do not introduce new birds, except through hatching or I consider hatchery chicks safe. Chickens can transmit diseases to each other. The less you expose them to potential pathogens or potential carrier birds the better. Practice good bio-security. Do not bring in pathogens on your clothing or especially shoes if you have visited where other chickens are.
Do not overfeed them. There are a couple of aspects to this. Do not give them a lot of special treats, they need a balanced diet. Giving them a few treats is fine, just do it in moderation Also do not look at how much the breed is supposed to weight. That's for best productivity or show birds, not for longevity. The more protein they eat the larger their body will be and more and bigger eggs they will lay. The more bigger eggs they lay the more likely they are to have medical problems with their internal laying equipment. The bigger they are the harder their heart and other organs have to work to maintain the body. If they get big enough they can injure themselves just hopping down from the roost. For some goals feeding them lots of protein or other nutrients is important, but not for longevity.
An obvious one, protect them from predators.
Regularly inspect for mites, lice, and worms. Treat them if appropriate.
That's it off the top of my head. Some of this is counter to good productivity or other goals. Predators, disease, parasites, or medical conditions will get most before they can get to 15 years old. That's really old for a chicken.