It’s just an estimate from scientists. Many of these ages have been established using a technique called bomb-pulse dating, which uses traces of atomic radiation we all carry to determine how old cells are. Cells are replaced on rotation not all at once. So they regenerate, hence it is not too late to attain a healthier body now with good nutrient based diet and lifestyle. 

Synergy Nutrition- Estimated lifespan of your body’s cells ©