I feel like it might come from the fact most relationships are pretty short before you are 24. Few people hold onto long lasting relationships by that age and few (at the time) short ones develop into anything reliable.

A former classmate of mine met a guy and got married after knowing him almost a year, like right out of highschool. Last I heard of her they went through a messy divorce couple of years later, which we all saw coming and tried telling her about.