Which planet would i be oldest on?

Answer 1

Interesting question. It turns out, gravity slows time. You would therefore age fastest on the planet with the least gravity. As we no longer consider Pluto a planet, that would mean you would be older on Mercury. Mercury, however, is closest to the sun, which also exerts some gravitational pull. Nevertheless, without working the math, I think I would still stick with Mercury.

If you and a twin were born at the same time on earth and Mercury, (earth being the heaviest of the four rocky inner worlds), my guess is that by the time you had each lived a century, the twin on Mercury would be a few seconds older.

Someone should do the calculations to confirm this--I'm just making wild estimates here.

A second way of looking at the question concerns the number of years you would have attained. Mercury orbits the sun 4 times for every earth orbit, so if you were ten earth years of age you would be 41.5 in Mercury years. The further from the sun, the longer it takes a planet to orbit. But aside from the gravitational/relativistic effect, you would still be about the same age.

Answer 2

You would be oldest on Mercury, as it takes the longest to complete one orbit around the sun due to its proximity to the sun and faster orbital speed compared to Earth.