Says who? Our best evidence says that muscle cells live for our whole life as well, only increasing/decreasing in size in response to exercise. Liver can regene

Says who? Our best evidence says that muscle cells live for our whole life as well, only increasing/decreasing in size in response to exercise. Liver can regenerate but I'm not aware of turnover or a death checkpoint for them. Same with the other solid organs. We've got lots of cells that are turning over continually, but plenty of long lived structures as well. Also, if it's related to cell turnover, you would expect cells which replicate through our life to have higher numbers of mutations compared with our initial zygotic state due to the potential for mutation at each point