How old is a 12-year-old on Mars?

About 6.38 Mars years old

On Earth we count years by Earth's orbit (about 365 days).Mars takes 686.98 Earth days per orbit, so someone who has lived 12 Earth years has completed fewer full orbits — hence a different "age."

A Mars day is only 40 minutes longer than Earth's - but you'll wait 6 months for summer to arrive!