How old is a 8-year-old on Neptune?

About 0.05 Neptune years old

On Earth we count years by Earth's orbit (about 365 days).Neptune takes 60182 Earth days per orbit, so someone who has lived 8 Earth years has completed fewer full orbits — hence a different "age."

Neptune takes 165 Earth years to orbit the sun. It has only completed ONE orbit since its discovery in 1846!