I'm pretty sure "getting old" has always been a big part of the collapse of civilizations.

I can think of few that were truly destroyed by unstoppable external forces (maybe the Byzantine Empire being attacked by Muslims?). Most of them seem to have rotted from the inside and were easily knocked over by opportunists (Rome). That said I'm no historian so examples would be interesting.

Also interestingly, America is 243 years old - 93 years to go! Sounds reasonable.