Open-plan offices require you to trust strangers in a way that most people aren't comfortable with. Being visible to someone while working (i.e. doing an activity where your income depends as much on how you are perceived while doing it as on what you actually do) is a form of mandated trust of others.

It doesn't come naturally to people whose life experiences involve violations of trust, embarrassing illnesses, or physical danger. (With age, the percentage of people who have such experiences increases, and approaches 100%.) You have to still have that sense of immortality to be OK in an open-plan office.

Also, having children makes people less trusting of strangers, for obvious biological reasons. Having a helpless being that you must protect at all costs makes people less OK with the blind trust in strangers you're supposed to have if you want to fit in to the "I'm at one with everyone, man!" California culture.

Older people are also not able to take as much abuse of their fight-or-flight systems, and pregnancy has a similar effect on women.

Even if those correlations hold true and have the effect you're describing (I'm far from convinced that's the case, but it's moot), it strains credulity by a mile to claim that this is some byzantine plot for "backdoor age/sex/income-discrimination". That would mean that those making decisions about open offices are 1) thinking through the complex and frankly tenuous connections that you're describing, 2) finding them credible, and 3) deciding that the collateral damage to other employees' productivity and happiness is somehow worth the weak effect this would have on furthering discrimination. There are an infinite number of ways to backdoor-discriminate on those bases that are not very noticeable, far more effective,