Implement a hidden or visible tag system that allows a user to define their interests or age.

When they then join a world, it will attempt to place them in an instance where there are people with similar interest or age tags.

This would serve the dual purpose of addressing user concerns with the amount of children in public lobbies while also increasing the satisfaction of the user with joining public instances as they will be more likely to find like-minded users.

To go along with this feature, it would make sense to have a list of popular user defined tags, so people can more easily find common interests.