Human cells can only divide about 50 times before they stop for good

In 1961, Leonard Hayflick discovered that normal human cells in culture divide a finite number of times, roughly 40 to 60, before entering senescence and halting permanently. This overturned decades of belief, based on a flawed 1912 experiment by Alexis Carrel, that cells could divide indefinitely under the right conditions.

— Leonard Hayflick, Paul Moorhead, The Serial Cultivation of Human Diploid Cell Strains — Experimental Cell Research, 1961