I've wondered if Feynman's counting rate -- <a href="http://calteches.library.caltech.edu/607/2/Feynman.pdf" rel="nofollow">http://calteches.library.caltech.edu/607/2/Feynman.pdf</a> -- varied with age.<p>(tl;dr While a grad student he got interested in this question of subjective time rates and started counting in his head while doing different things. The story doesn't say if he ever tried it again decades later. Unfortunately I don't remember timing myself the first time I read it -- enough time's gone by that I could have looked for a change now.)