A writer once speculated that this is because as people age the deterioration in their physical health, and their increasing inability to comprehend the behavior of the young they are farther and farther removed from, is confused with a deterioration in society generally.
Socrates supposedly complained about the insufferance of the youth of his day, who lacked the respect for elders that his generation had.
America does seem at once crass and materialistic and strong and indomitable.
These two observations are different flowers of the same seed.
There is some truth in your observation that bodily decreptitude causes pessimism, but a hard life can be a school of virtue, though it isn't necessarily one. Every generation does have anxieties about future ones, but I wouldn't necessarily write off those anxieties. At some point in the life of empires, those fears are shown to be quite realistic and proven by the inability to handle certain crisis.
If we had to give it all we had, to commit much more of our resources to war, as we did in 1941, would we be able to do as well as that generation did? One can make the case that being used to a comparatively soft life and having things done for us has spoiled us, or that skepticism may be a sign of civilization, but it also makes it hard for civilizations to defend themselves.