The average life expectancy as recently as 1900 (not 1776) was about 47 years. The average Founding Father at age 43 would have been relatively old in light of these statistics. Other than George Washington, how many Founding Fathers actually took up arms in battle? The Founding Fathers were more of a spiritual impetus than fighting force. They were the gray-haired instigators of an earlier time.
While it is true that people did not live into their 90's on a regular basis, like now, it is also true that the GREATEST reason average ages SEEMED lower was NOT due to early death. It was mainly due to infant mortality. 40's were not out of the ordinary at all. They were not "gray-haired" with one foot in the grave. MANY were out in the field, fighting, killing off bad guys.
George was only the leader, there were people of all ages fighting.