The actual paper the number comes from (Fate of Empires by John Glubb) is complete bullshit, though. Even the cherry-picked examples it uses, which are limited strictly to the surroundings of the Mediterranean, don't use any kind of consistent criteria for when an empire starts or ends. He tries to count "Alexander (and his successors)" as one coherent entity and then picks an end year in which all of them had either already collapsed long ago or would not do so for many decades to come. He cuts centuries off of the Roman Empire's lifespan by just saying that the empire was unstable and getting invaded a lot (and ignoring the Eastern Empire entirely). HIs reckoning of the "Arab Empire" includes three separate caliphates, and the end date isn't even the actual end of any of them

Other than that, no, it does not attempt to find an average in the sense of a mean lifespan. It actually does argue that 250 years for an empire can be compared to a human living 70 years.

But if the average hides a huge range of datapoints, it becomes less useful.

Then it wouldn't be reasonable to assume the US would collapse right at the average (mean) though. If the majority of empires collapsed at the same age (the mode) it would be different, but the mean tells you very little about when any particular empire will collapse.

The mean number of children per household is a decimal, that doesn't mean any households have partial children.