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.

Because teenagers often exclude, harass, and verbally abuse others in the same age group for no real reason. If you're a teen who often ends up on the receiving end of that, then interacting with older people may be the only way you can have a normal conversation. Talking to adults is a last defense against missing a stage of social development and developing depression and social phobia.

That doesn't mean that you are personally required to talk to teens, but cutting them off from adult society completely is cruel.

And it is possible for a teen and an adult to share an interest—baseball, or sewing, or entomology, or whatever. Some people like talking to others who are enthusiastic about the same subject, even if that person is young and "still learning". Some adults even enjoy teaching.