An Age Line is an enchantment used to prevent people above or below a certain age accessing a certain object or place.
From the Story
Discovered in Book 4, Chapter 16, The Goblet of Fire"Dumbledore announces that an Age Line is to be placed around the Goblet of Fire, to prevent students under the age of seventeen from attempting to enter the Triwizard Tournament.
The Age Line is visible on the floor surrounding the Goblet of Fire; a thin, gold line which forms a circle ten feet around it in every direction. The Age Line physically ejects underage students who attempt to cross it; Fred and George Weasley are hurled out of the golden circle as though thrown by an invisible shot-putter, landing ten feet away from the Line. The underage victims of the Age Line sprout long white beards as a consequence of trying to get past it."