Mine are still small, so I don't let them stay up too late. I can tell that they still need a good ten hours of sleep, because they just wake much later in the morning if they have a late night. So I let them stay up if they're in the middle of something really fun - usually getting filthy in the garden with their friend from across the back. But otherwise I try to get them to bed at a decent hour still, albeit a little later than school nights, as I need my own evenings even more in the summer because they're exhausting kids! They don't stop moving and shouting and questioning from the moment they get up until the moment they fall asleep! I've got to have a couple of quiet hours before I go to sleep myself, man!
My eldest, who's only eight, stays up a little longer than his brothers, because he has his own room and he's no trouble at all at bedtime - he just reads quietly, or does some drawing or writing in his diary/journal. That's often his most productive time of day, because it's his quiet time as well, and he gets peace from his brothers.
Nice question :)