The phrase "eighteenth birthday" refers to a special day when someone turns 18 years old. It is often seen as a milestone because it marks the transition from being a teenager to becoming an adult. Full definition
We may play the date of our wedding anniversary in the lottery, remember that magnificent eighteenth birthday when we got to drive our father's car for the first time, and long to lose that extra ten pounds, but the number that most impacts our life — both in what we decide and in what others decide about us — is our credit score. (ovationcredit.com)
I would love to SPARKLE in beauty at my sons eighteenth birthday bash. (fakefabulous.com)
have six children, save all their money to go to Mecca and their daughters will marry right after their eighteenth birthday to some one of their faith and immediately begin having six children in her own two bedroom apartment and they will save all their money to go to Mecca. (religion.blogs.cnn.com)