The meaning of "AS"

… Finally, homebuyers no longer want a second floor. They want their new houses to have only one floor without stairs. As Americans get older, they worry about falling down the stairs and hurting themselves.
I’d like to know the meaning of “as” in the paragraph. Does it mean “when”, “because”, or what?
Thank you very much!

It`s a comparison that has a strong sense of finality. It can not fully be translated into “because” though. You actually have to look at two points in there, “as” and "older.
The “older” a comparative:
The “as” like a comparative conjunction for a subordinated clause.

The meaning of ‘as’ is here closer to ‘while’:
If something happens (here: Americans get older),
something else happens (here: they worry about falling down the stairs) -
both actions happen at the same time.

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“As” is a tricky word. It can have a lot of different (although, arguably, related) meanings.
In this case, it is used to express a relationship between two unfolding processes:
Over time, American individuals (as all others) get older. It’s a gradual process that unfolds over time. At the same time, they begin to worry about falling down stairs. That is, the older they become, the more they worry [and/or the more likely they are to start worrying about it]. The sentence alludes to a parallel development of both processes which in turn points at the likelihood that former might be the cause of the latter