What chemical{s} in our bodies cause us to age & get old?

Question

what causes us to age? I want to know this from a science point of view.

Answer 1

Telomeres are considered to be the culprit.  

Telomeres are gene sequences at the end of chromosome strings.  As cells reproduce, these sequences become shorter and shorter.

At a certain tipping point, a specific shortness of the telomere sequence creates a signal to other genetic components within the cell to destroy the cell itself.

Of course, the process in not that simple.  I am just providing a high level, metaphorical explanation based on my specific knowledge set.