At the end of human chromosomes, there's a repeating but non-expressive DNA segment, which "loses a piece" with each replication, known as the "telomere."
The normal length of a human telomere is around 8000-10000bp, and in most differentiated somatic cells, about 50-100bp is lost with each division.
Normal somatic cells can divide an average of 50 times, and each division cycle is about 2.4 years. According to this calculation, the normal human lifespan is around 120 years, which is commonly referred to as the "Hayflick limit."
Additionally, a magical substance called "telomerase" can maintain the length of telomeres, indirectly extending lifespan.
Unfortunately, telomerase activity is only maintained in germ cells, stem cells, and 90% of cancer cells, and normal cells don't enjoy this benefit at all.
