[inglor_cz]

For a big power, the US is relatively young. As a country, it has been around for 250 years, but as a world power, mere 100. That is on the young side when compared to other historic empires.

The Roman empire in the 1st century AD looked remarkably stable and rich. You wouldn't be able to prophesize the crisis of the 3rd century from the status quo in Hadrian's times.

[nine_k]

Which post-medieval states had a much longer tenure as a comparable world power? Spain kept for about two centuries, and Britain maybe for somehow more. Can't readily tell about China, but it did dominate its vicinity both culturally, economically, and militarily for quite some time, before a long and miserable decline that lasted until late 20th century.

[inglor_cz]

France, the UK, Spain, Portugal, the Ottoman Empire, Russia, I would say. All clocked at 250+.

Of course, we may split a lot of hairs discussing what "comparable" means. Certainly all of those were great powers, controlling distant shores and distant nations for long generations.