Why is that so bad? As a kid I really appreciated participating in mixed-age discussions on many topics. I view that as part of what it means to grow into a "young adult."
Too often I think we (North American society) assume that school, with all it's rigorous age separation, gives kids the space and instruction they need to do well in the world but inevitably we get 18 year olds with no awareness of how the world functions beyond themselves... because they've only ever dealt with people of the same age.
The world is a diverse place; ideologically, racially, and in age. We, adults, need to be comfortable communicating with both children and legal minors because they'll be future citizens of the world [added in edit:] and they need to learn those skills too.
Overall, we keep trying to model a world that filters it's own interactions towards children, which is flawed to begin with, but at some point people stop being children, and where does that leave them w.r.t. their expectations of others? If you've never had to consider that an adult might act in bad faith because your world has been so sanitized, are you prepared for a world with bad actors in it?