"An Age Line! ...Well, that should be fooled by an Aging Potion, shouldn't it? And once your name's in that goblet, you're laughing - it can't tell whether you're seventeen or not!"
-- Fred Weasley, wrong about the Age Line (GF16)
Effect: Causes the person drinking it to grow older. The more Ageing Potion one drinks, the more one ages.
References from the canon
Fred Weasley, George Weasley, and Lee Jordan took a few drops of Ageing Potion in an attempt to fool the Age Line around the Goblet of Fire into thinking that they were a few months older. The Line wasn't fooled. They were thrown back out of the circle, then sprouted full beards (GF12).
NOTE: In the US editions, this name of this potion is spelled 'Aging'.