Well heart rate is like a variable, if someone is bigger blood has to flow farther, and takes more pressure to deliver the blood. As you age, cholesterol accumulates in arteries and makes each pump of the heart less efficient, and someone with good fitness needs less beats per minute to deliver all the blood/oxygen they need. Michael Phelps, for example, probably has a heart rate of less than 40 BPM; Whereas as person of the same age that is less involved in an exercise regiment may have a heart rate around 80 BPM.
