Age Line Charm
This spell creates a barrier which detects the age of anyone who is trying to cross it. Those who are too young are immediately ejected.
Dumbledore created one in the Goblet of Fire where anyone under the age of 17 wasn't allowed to cross. Fred and George Weasley had tried to 'fool' the Charm by using an Aging Potion but were inevitably thrown from the circle and managed to sprout a long white beard as a result. Some Age lines can be fooled by Aging potions, but this particular Age Line was cast by Dumbledore, who was a very magically potent wizard and knew how to stop all under-17s.