I'm not going to delve into any evidence in this post, I'm merely speculating on the motivations of young-earthers.
I used to be a young-earther myself (then I realised how stupid it was), but the only reason I believed the earth was young, is because that's what I was lead to believe by adults when I was younger. I had at least one creationist teacher when I was in primary school, influences from other adults, and then of course the Kent Hovind videos and articles that I was so fond of. I only believed in a young earth because I was told it was so, and since I didn't know any better I accepted this explanation. What I can't understand is the people who propagate these fairy-tales about a young earth.
The evidence overwhelmingly supports an earth that is ~4.5billion years old, and for adults to hold young earth beliefs requires an enormous amount of self-deceit.
Perhaps I'll take a more in depth look at this later.