Reason for teens to rent a limo

PROM

Think of a formal high school dance where students often arrive in style—it’s a milestone event marked by dresses, tuxes, and group photos.

Teens often rent limos for special occasions, and "prom" is the biggest one—a formal high school dance marking the end of the year. The clue plays on the idea of splurging on luxury transportation for a milestone event. Since prom is a well-known tradition, the answer fits naturally without needing extra wordplay.