The first line in the movie is "When I was 16, I fell in love." But when Cameron Diaz and Ben Stiller meet up in Florida, she says, "I haven't seen you since senior prom." If you're 16, then you're not a senior yet, unless everyone skipped a grade.
He might have fallen in love with her at 16, and known her for a long time after that, before he invited her to the prom. I fell in love with her myself in The Mask.