the cells in the clone were fresh. the DNA was 6 years old.
and stopping heart/lungs may not be the only cause of death. decapitation will usually kill a person. so will gigantic loss of blood. large blow to the brain. many other things.
scientists are now researching a part of DNA, called Telomerase. it's a bunch of junk DNA at the end, most likely a buffer zone of some kind, that has a little bit shaved off every time the DNA replicates. after conception until the first several years of childhood, these ends don't shorten with the help of some enzymes. this enzyme stops being produced after a while.
the little buffer zones may help keep the DNA in good shape, but as we get older, there may be ends of the useful DNA being shaved off and the body doesn't function as well without all the information. also, after 75 years or so of damage due to radiation, poisoning, stuff like that DNA gets damaged.