Given an age in seconds, calculate how old someone would be on:
So if you were told someone were 1,000,000,000 seconds old, you should be able to say that they're 31.69 Earth-years old.
If you're wondering why Pluto didn't make the cut, go watch this YouTube video.
Note: The actual length of one complete orbit of the Earth around the sun is closer to 365.256 days (1 sidereal year). The Gregorian calendar has, on average, 365.2425 days. While not entirely accurate, 365.25 is the value used in this exercise. See Year on Wikipedia for more ways to measure a year.