I may be misinterpreting their comment but I initially read elder as "elderly people" and not just "people older than them" and that the point was about how adults refer to them by first name unless it's in a professional environment (e.g. doctor or government title)

For kids at least formality still mostly applies, I'm Gen Z in northeast US and I grew up calling every adult by Mr./Ms. X, with the only exceptions being family. This could be different in other parts of the US though, I don't mean to generalize.