Purebreds do not generally live as long for genetic reasons (usually related to successive inbreeding). Some breeds tend to live longer than others from genetically-inherited longevity. Keeping your pet on a healthy diet with plenty of exercise and regular visits to the vet and dentist will keep your pet living a much longer, healthier life in general, so good care of your cat is essential.
Longevity depends on both genetics and environment just like for people. Some Maine Coons will live just as long as any mixed breed and even longer. Generally speaking purebred animals are more prone to certain health problems due to their genetic makeup which may not be as diverse as some of the mixed breeds. However, this is not always the case as many mixed breed cats come from heavily inbred wild colonies (farms etc). Maine Coons can live for approximately 8 to 18 years of age, just like any other breed of cat.