Why people don't live forever?

Human beings have parts that wear out. Some say that every day a person lives, starting at birth, is one day closer to their death--and it's true. By the time a person reaches 40-50 years old, some parts begin to show wear and tear. We have: achy bones; sore muscles; trouble digesting foods; gallbladders don't function as well; our vision and hearing declines; even our skin starts to get drier and less elastic. By the time people are 50-60 years old, most are taking 5 or more medications.