Or diet and exercise, which need not be uncorrelated with wealth, but something tells me that places like Sardinia, Okinawa and Nicaragua haven't historically been that wealthy. Especially when you consider that to have the most centenarians now, those centenarians must have been born in the first quarter of the 20th century, 100+ years ago. Whatever effect blue zones have, it's not likely to be an entirely recent one; People don't tend to suddenly gain huge amounts of life expectancy in their middle age.

(Of course, this is all presupposing that the number of centenarians in those areas is not greatly exaggerated, and that there hasn't been a massive migration of centenarians from other areas to the blue zones.)

Nicoya (Costa Rica) is not Nicaragua

Whoops, you're right. The picture I saw was relatively low res and it kind looked like it was Nicaragua.

Or genetic mutations (which also may interact with specific lifestyle factors). At least one such mutation was identified in an Italian family that is seemingly immune to atherosclerosis [1], and the "blue zone" populations tend to be relatively small and isolated (at least before the industrial revolution).

[1] https://en.wikipedia.org/wiki/ApoA-1_Milano

On your parenthetical, here's a hypothesis. It's kinda hard to be old in rural areas(no way to get around, limited care available, etc), which tend to be inland, whereas cities tend to be coastal or situated on a river.

So I could definitely see it as plausible that the older people get, the more people move into cities, which coincidentally involves moving closer to water.