I think this sort of thing varies tremendously with the individual child.
My DD was fine with (and insisted on answers about) pretty heavy-duty topics at a very young age. Death, war, poverty, genocide, rape, intellectual disability, hidden/non-hidden disabling conditions, and mental illness were all conversations that I know I had with her in all kinds of settings over a period of many years, and starting when she was two or three. We'd covered all of that by the time that she was 8 or 9.
I certainly HOPE that nobody eavesdropping was thinking that I should not have those conversations with my daughter, or that they were things that shouldn't be discussed in public. We don't differentiate academic topics of discussion that way in our family. Private, personal topics, sure-- there is a sense of needing privacy.
But I don't spend time worrying over my DD overhearing someone else discuss something I don't want her thinking about. I can't imagine having a child that I'd need to worry about that. It must be very hard to shelter a sensitive soul like that from just casual inputs in public. I hadn't thought about it before now.
While I agree that emotional readiness to process these topics varies by child and may be academic to them, it may not be an academic topic to whoever is in listening vicinity in public. When the child has the emotional readiness, sensitivity and maturity on when, where and with whom these topics are appropriate to discuss is another gauge of how much information to share.
