"To ensure that no underage student yields to temptation. ...I will be drawing an Age Line around the Goblet of Fire once it has been placed in the entrance hall. Nobody under the age of seventeen will be able to cross this line."
-- Albus Dumbledore (GF16)
A thin golden line drawn on a floor which affects anyone who crosses it if they are too young.
References from the canon
Albus Dumbledore drew an Age Line around the Goblet of Fire to keep away anyone who was not yet seventeen years old. When the Weasley twins, who had taken an Ageing Potion to try to fool it, crossed over the Age Line, they were thrown back and sprouted long white beards (GF12).
Although not shown in canon, one could presumably create an Age Line which could prevent someone to cross who was older than a given age.