Aging

Why do we age? We age because we are a multicellular organism with physiologic functions, and as time progresses, those functions deteriorate. It happens to every living thing. But what causes it? Many things can contribute to the aging process, some even accelerating it. Many theories exist. Oxidative stress is believed to play a role as well as genetic instability and just general wear and tear. Diseases like cancer or autoimmune diseases have been thought to speed up the cellular aging process. Abusing substances like drugs and alcohol and smoking can cause stress to the body and may contribute to premature aging as well. We can help our bodies by consuming healthy foods, getting exercise, and avoiding damaging substances. Many people now live much longer than people even just 100 years ago due to improvements in disease control and nutrition. 

https://www.ncbi.nlm.nih.gov/books/NBK10041/