1. You're not invited to my birthday
  2. My dad could beat up your dad
  3. I know you are but what am I?
  4. Up your butt and around the corner
  5. Homo/gay wad/faggot
    We are clearly in the middle school years
  6. Slut/Bitch
    Still in middle school
  7. Virgin!
    High school. Duh.
  8. That was so trite
    We are now in college. Specifically a liberal arts college.
  9. You have daddy issues
  10. You're so insecure
  11. What are you doing with your life?
  12. You spoil them/are too strict/don't discipline them enough/are too hard on them
  13. You're just like your mother/father
  14. ... Then what? Do old people insult one another or are they just racist?
  15. Poopy face!
    One of the earliest. A precursor to the use of "shithead" later in life.
    Suggested by @dball
  16. I'm rubber you're glue whatever you say bounces off of me and sticks to you
    Suggested by @cabramow
  17. That's so funny I forgot to laugh
    Suggested by @dandylyons
  18. "Got a match?" ... "Yeah, my ass and your face."
    Circa middle school
    Suggested by @fats
  19. What are you looking at, dicknose (appropriate for ages 5-97)
    Suggested by @jcspesh