Muddy pitches, heavy leather balls, much lower scientific knowledge in diet and fitness, leniency in tackling and general rough play, and differences in lifestyle all play a part. Also remember that many players of the time were doing manual labour up until they turned pro, whereas young players today are practically raised inside academies with nothing to think about except football. A lot of footballers back in the day physically regressed by the time they turned 30, but now we see that the typical peak age of strikers appear to have extended beyond that age.