The lower age limit for one's girl- or boyfriend, below which the relationship becomes creepy. Found by the following formula:
creepiness limit = 0.5x(your age) + 6
Breaks down when you're below age 13 or over 65, approximately.
Eg. the creepiness limit for a 30-year-old would be 21, but the creepiness limit for an 18-year-old would be 15.