Hi Canyon,
We think, based on studies of rocks from the Moon and Earth that the Sun is a bit under 5 billion years old.
We also think, based on modeling and observing stars that it has about another 5 billion years to go until it becomes a "red giant" star, expending out to near the orbit of the Earth. This would mark the end of the Sun's life as what yo might consider a middle aged star. It will spend a while as a red giant, then we expect it to loose its outer layers, leaving a white dwarf star which will slowly cool over many billion years.
You can read more about the life of a star like the Sun here:
http://www.windows2univer...ff/tourstars_1a.html
http://www.windows2univer...ff/tourstars_29.html
best,
Terry