We have two boys, aged 12 and 15. We live far from family, and they only have one cousin anyway. Neither DH nor I are close to any of our cousins at all. We see grandparents a couple of times a year, as a family. With my sister (who doesn't have kids), she usually sees the boys at my parents place - she and her partner aren't into doing 'family / kid-friendly' stuff, so DH and I tend to see them without the children.
When I was growing up, I think I actively avoiding family events from age 14 - basically as soon as I was old enough to be left home alone, that's what I wanted to do. I'm pretty sure that my parents friend's children also started staying home around the same age.
So now we have several family friends who have children the same age as my children, basically friends that I made when all our children were babies. We've socialised together as families all this time quite happily - usually for a long lunch that stretches into the afternoon. However, our oldest children are now 15 / 16 and I'm starting to wonder how much longer we can 'make' them spend whole days together at our long lunches. They are all friends but they don't hang out together, but also they've never objected to being together all day.
Do you include your children in socialising with friends? Do you expect them to join in / host the visiting teens? At what age does it stop, do you think?