How old is a 21-year-old on Mercury?

About 87.19 Mercury years old

On Earth we count years by Earth's orbit (365 days).Mercury takes 87.969 Earth days per orbit, so someone who has lived 21 Earth years has completed fewer full orbits — hence a different "age."

On Mercury, a year is shorter than a day. You'd have 2 birthdays during a single sunrise-to-sunrise!