This one might have a regional element to it based on how late the days are. For example where I'm at it won't quite be sunset at 9pm, and each phase of twilight will have around an hour to itself. Around now would be about an appropriate time for a young person to make their way home so they get back before dark. In comparison it looks like Houston will be about halfway through nautical twilight and basically be completely dark by 9pm (nautical twilight ends at 920 there).

So yeah, your gauge of what 9pm means might not be accurate for the situation! I doubt you were imagining 9pm being bright and an hour or two before its too dark for a young person to be out riding a bike, but that could well be the case