The answer that I give might be a bit contradicting for some people. But according to me the aging process happens due to few factors.
Sleep: Sleep is the major contributor for aging. If a person is sleeping a lot, there is a high possibility that aging process can be faster. Body growth and recovery happens only during sleep. When a person undergoes lots of wear and tear throughout the day time, it is only during sleep the body will try to heal. If we see the kids, we will be able to see that they grow faster if they get a good sleep.
Breathing: The next important contributor for aging is breathing. An ancient writer called Thirumoolar who is believed to have lived for 3000 years mentions in one of his songs that a perfect breathing system is important for slowing down the aging process. In order to live longer we have to breathe less. The more we breathe we are losing many years in our life. That is the reason why Tortoise lives longer and a dog has shorter lifespan.