Dogs live 12 to 15 years, and the theory says that we can compare the Dog age with human age, multiplying the dog age * 7.
Usually all animals get the maduration of his bodies in a certain time, and they live Maduration Age * 7. That means, if a dog develops his body in 2 years, then he will live 2 * 7 = 14 years approx. Humans develop our bodies in 20 years, that means that we should live 140 years, but we don't live that amount of time.
About vitamin c, I read that humans were able to build Vit C long time ago. But now, we cannot. But we can build Vit D with the aid of the Sun.
I also read somewhere that insulin or sugar, and Vit C are very similar and when we take too much sugar we are destroying the Vit C we store in our bodies and we're destroying the collagen of the body and the arteries and veins loss a lot of elasticity due to the lost of the collagen.
Sharks, in example cannot develop cancer. The reason is cancer and regeneration potential are opposite. I mean, when we have less regeration potential we can have more cancer and viceversa. The sharks are developing teeth again and again. Humans cannot.