How cells stop working
Living things are made of cells that constantly work to keep the organism alive. Over time, these cells get damaged from normal activities like breathing and processing food. The body has repair systems to fix some damage, but these systems eventually wear out. When damage builds up faster than the body can fix it, cells stop working properly and die.
The limit on cell division
Most cells can only divide and make copies of themselves about 50 to 70 times before they stop dividing. This limit is controlled by telomeres, which are protective caps on the ends of DNA. Each time a cell divides, the telomeres get shorter. When telomeres become too short, the cell can no longer divide and eventually dies. This is called the Hayflick limit.
Accumulation of damage
Throughout life, cells experience damage from many sources including sunlight, radiation, chemicals, and normal chemical reactions inside the body. DNA can get mutated, proteins can become misshapen, and cellular structures can break down. While cells have repair mechanisms, they are not perfect. Over decades, small amounts of uncorrected damage build up in cells and tissues, making organs work less efficiently.
Organ system failure
As cells die and are replaced more slowly, organs begin to lose function. The heart pumps less efficiently, the brain processes information more slowly, and the immune system becomes weaker. Eventually, one or more major organs fail completely, which stops the entire body from functioning. When critical organs like the heart or brain stop working, death occurs.
Natural program in living things
Some scientists believe organisms have a built-in aging program that limits lifespan. This may have evolved because it helps species adapt and make room for new generations. Different species have vastly different lifespans, from insects that live days to tortoises that live over 100 years, but all eventually reach their natural end. 
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