He won't be the first heir apparent to have hung on for a long time, Edward VII had nine brief years when he followed his mother, Queen Victoria. Similarly George !V too pretty short reign. He got a stab at Prince Regent for quite a while I believe whilst his father George III, suffering from Porphyria, mistaken for madness was off the scene, shut away being bled or whatever the crazy quacks did then not really knowing how to treat it.
I don't think Charles looks as healthy and robust as he could for someone of his age, for example he is very florid and seems to suffer from puffy fingers.