People have identified over fifty different things which may have helped cause the empire to decline. From hyper-inflation, caused by massive imports but low exports, to lead poisoning, the failing of democratic accoutability, to plagues. And maybe that it simply grew itself to death, the bigger and more diverse it became the more it became filled with people who cared nothing for it, but far more for their local leaders and it failed to propagandize itself, well enough to make its thousands of new members care about it.

That last is an interesting one, because of course Rome was the first major republican empire. Not an empire made by one wave of conquest led by a single king, but one which grew by stages, adopting the ruling classes of other nations as citizens, as often as not. Thereby making conquest easier, (many people even asked to be absorbed ), but becoming ever more culturally diverse and also, ever more divided against itself. That may seem to be an anti-liberal, anti-multicultural, message, but it is only so because the Romans also failed in their education/propaganda needed to explain the empire to its diverse subjects, and it also failed to reward people by preventing a deepening rich poor divide. (Yes that may sound familiar.)

Three centuries ago, Gibbon was famously fond of the theory that Christianity was the cause, but while Christianity may have contributed a little, most people would agree today that gIbbons view was far too simplistic.

I've read that Rome was founded in the 700s BCE and the 'Roman Empire' lasted until the 400s CE. That's over 1000 years.
Could it be the fall of the Roman Empire was caused by old age? Even societies are subject to a variation of the gambler's ruin principle. A player's fortune rises and falls over time. But when it falls below zero, the game ends -- there's no coming back (from death?).

No. The Empire split in two, for egotistical reasons. The Eastern (Byzantine) side embraced Christianity, and as a result both won and flourished, until Isla'am overwhelmed it. The Western side had to re-invent itself as The holy Roman Empire, a moral authority, because Rome was militarily defenseless against waves of Northern invaders. The actual empire collapsed because decadence had pervaded all authority. .... and also, because fewer and fewer Romans were being born, despite creating all sorts of orgiastic festivals to encourage procreation. Little did they realise their love of lead was to blame. Lead water pipes, white lead (lead oxide) to sweeten wines, etc.

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