Do the elderly have similar infrastructure requirements though? Like, generic infrastructure like highways or rail lines is a requirement and doesn't really change much as it's required for transporting of goods etc, but lots of local infrastructure seems to be something you're leaning on a lot when you're younger. It's an exaggeration of course, but if you keep the groceries coming and the TV program changing, the elderly will be somewhat content.
But even then, don't we have infrastructure productivity improvements, too? Building something these days requires more skilled workers, but much fewer.
Bridges, highways, sewers etc all still need maintenance even if there are less people using them. This is an increasingly large problem in Japan, which built lots of hugely unprofitable infrastructure during the Bubble years (bullet trains to the prime minister's rural home town etc) and is still addicted to grandiose projects as a form of rural subsidy/kickback.
Don't infrastructure projects scale well with regards to costs? I assumed that e.g. building a three lane highway isn't 50% more expensive than building two lanes, but maybe it even complicates things more as you run into more issues the more space you take up (can't squeeze through gaps, more attack surface for Godzilla and whatnot).
Isn't infrastructure profiting from automation as much? Health & elderly care has severe limits there until we get to very advanced robots in a few generations, but infrastructure feels easier.
Scaling only gets you so far. The maglev Chuo Shinkansen currently under construction between Tokyo and Nagoya involves around 250 km of tunnels punched through Japan's mountainous backbone and has a price tag of US$64 billion (and continually climbing).
Still, the real problem is not building new things, but maintaining old ones.
But even then, don't we have infrastructure productivity improvements, too? Building something these days requires more skilled workers, but much fewer.