I'm assuming you've done a personal study on the topic. Have you found that both parties to the ass-picnic have to be below 30 or does just one dictate the practice or lack thereof? Is there a direct correlation between butt-tongueing and date of birth, or do people grow out of it/continue the practice? Is 1990 the demarcation point, and if so was it 1989 last year and will be 1991 next year or will it remain static?
