How old is a 5-year-old on Mercury?

About 20.76 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 5 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!