Quote Originally Posted by chick View Post
I only wished to refine your "4 to 6 billion years" estimate

The early universe formed only stars of hydrogen and helium. Those stars had to go through a life cycle to produce heavier elements in their supernova destructions. Those heavier elements had to rejoin into second generation stars, and that was the earliest that solid planets could form. Those planets formed of smaller pieces of solid matter called asteroids (chunks of rock in space formed themselves by accretion in the second generation star disk). The oldest of those are about 4.5 BY, suggesting that to be the earliest that solid planets could form.

Of course, in fantasy, anything you want to be true can be ....
Actually (still staying outside of fantasy where almost everything can happen), the life duration of a star is inversely proportional to the mass of the star squared (Life ~ k .(M/M0)^ -2.5).
So stars of a mass say 10 times our Sun which are precisely those that will go supernova live typically only a few dozens millions of years. On the other hand stars of mass less than our Sun's live a few dozens of billions of years.
That means that already 100 millions years after the birth of the Universe there have been enough exploded Supernovas to create some planetary systems similar to ours.

For this fantasy world there can't be any estimate of its age if one doesn't know something about its Sun. If it is a small Sun it may be as old as the Universe, up to 15 billions years (assuming the Universe is 15 billions years old) , if it is a huge Sun then it may be less than a billion.
For our Earth we have a small and relatively young Sun so that the heavy atoms around us are results of N generations of Supernovas, N being potentially very large.