Our place has no “rules” about boys and girls but the general trend is once they hit 10 they start to separate. My teen sons will say hello to someone they knew as an 8 year old, but not join them at the table in the pizza store. As far as adults, if your spouse is there, you can join the circle. If not (or if the spouse needs to leave) you leave. Its not rude, its just the gedarim we set up to make sure no accidents happen.
Its called using your judgement. We use it, kids use it.