[Thespian2]

Back of the envelope calculation. Say you are really tall, 2 meters. That's "only" 2000 millimeters. So even additively, that's still only 0.00000000000002% difference between your head and feet. Let's say our two meter giant lives 100 years. That's a bit over 3 billion seconds, and for their entire life, their head and feet continue to age differently at this rate. If I haven't dropped a decimal point somewhere, that's still only a life time difference of less than a microsecond (6x10^-7).

So "adds up quickly" is relative (heh) in this case, and inconsequential in absolute terms, at human scale.