not true by the way). But my favorite is the constant heartbeat theory.
This idea is actually quite old - one paper suggests it came to prominence in the late 1800's industrial revolution with the observation that machines that ran fast wore out more quickly. It then became 'supported' by observations across species - bigger animals live longer and also have lower heart rates. Perhaps we all have a finite number of heartbeats and exercising will only use them up faster. Oh noes. However even if this were true (which it isn't) it would not be an argument against exercise because one consequence of being fit is a lower resting heart rate. You don't need to lower it much to have the reduced beats in the 22-23 hours a day you aren't exercising more than compensate for the hour or two you do.
Although the initial theory proposed a constant number of heart beats this became refined to suggest that it's the
speed at which an organism processes oxygen that matters. There is evidence,
when comparing species, that creatures with faster oxygen metabolisms
die younger. Tiny mammals with rapid heartbeats metabolize oxygen
quickly and have short lifespans. Tortoises, on the other hand,
metabolize oxygen very slowly and have long lifespans. The free-radical theory of ageing provides a potential mechanism that links
metabolism to ageing phenomena, since oxygen free radicals are formed as a
by-product of oxidative phosphorylation.
The debate rumbles on. One of the complexities is that even if a relationship exists between species we wouldn't necessarily expect it to be the same within a species. When we look within a species it gets very complicated. here's a summary from a 2005 paper in the Journal of Experimental Biology (Body size, energy metabolism and lifespan):
The observed patterns of association between rates of energy metabolism and
the rates of ageing (or lifespans) of animals within species include all the
potential patterns of association - positive, negative and not significant.