There isn't a global rate, mitotis rate depends on the cell type and many other inputs.

For example, see Table 1: https://book.bionumbers.org/how-quickly-do-different-cells-i...

You /could/ compute a global mean or median mitosis rate, and show how it changes/doesn't change with age, but it wouldn't say very much biologically. A narrower analysis that considers cell type and other context could be meaningful.