That's a completely misunderstood figure. If most adults were dying before 35, then why would they set the minimum age to run for President at 36. That would be like us setting it at 80 since the current life expectancy is slightly over 79.Alas ... the average life expectancy in the US was only 35 years back then
The age at which people "die of old age" has been remarkably steady throughout recorded history, with people living to be 100 years rare, but far from unknown -- just like it is today. The low life expectancy "from birth" has always been dominated by infant mortality. Even if you look at data for life expectancy for people that reach just one year old, it gets much closer to today's numbers. Conversely, if you look at the remaining life expectancy once someone reaches 25, there haven't been major improvements over time.