What benefit does it do either party for under 16s to have direct interactions with random adults in their 30s/40s+?

For one example, if they're trans and find info about diy HRT younger than they would have otherwise and are spared years of irreversible changes and dysphoria.

Or, you know, just that they're people who should have a voice and ears and not be barred from the wider societal conversation from everything about TV shows to national politics to educational policies, that they should have a voice in, and are indeed pubescent and more mature than actual children at thinking.

To engage with other people, practicising their online interaction skills, as they're not going to suddenly be magically more able to resist disinformation and stuff at 16 because of a few months of brain development, the more important thing is practice and being exposed to online content.