Well, I can't speak for others, but I believe the earth is very young, about 6000 years old, and I do not believe in the Gap theory.
That said, one of the most difficult problems for young-earth creationists to explain is how starlight from stars and galaxies many millions of light years from earth could be seen at present. I thought on this for a long time. Now certainly God could have made the light to be here instantly, this is the appearance of age theory. And there is some substance to this, when God made Adam and Eve they probably looked like full grown adults, probably at least in their mid-twenties on the day they were created. So God could have made the light just be here.
What bothered me about this is that it makes God seem misleading. The great distance of the stars and galaxies gives the strong impression that the earth is indeed many billions of years old.
But I always believe true science will agree with the Bible. And now that much evidence is coming in that the speed of light was many millions or billions of times faster just several thousand years ago (in earth orbital time), the problem is solved. Light would have arrived here almost instantly, even from many billions of light years distance.
Here is a brief but good article on the subject.
http://www.worldnetdaily.com/news/ar...TICLE_ID=39733