There wasn't an entire "boom" generation of banal revolutionary-in-their-minds people in their 50s and 60s hanging around gumming up the works and doing their damnedest to prevent any progress whatsoever. So in that sense life was different for 38- and 40yo people than it is now.
No one alive now would have noticed. We are comparing the life expectancies of the present day to those of 1762. At 38, Kant would have already outlived most of his contemporaries.
People in the 1700's didn't all die in their thirties. Kant lived to 79. As for contemporary philosophers, Leibniz lived to 70, Hume to 65, Rousseau to 66, Adam Smith 67 and Voltaire 83. Average life expectancy was pulled down by infant mortality, but plenty of people lived long lives.
