I moved a young about 5 foot green ash tree from the woods in our back yard into our front yard 2 years ago. It's now about 10 feet tall. I hadn't heard about the emerald ash borer. Its growing like gang busters. Should I just get rid of it before it gets to big or should I keep it? Also, if I keep it should it be treated right away or do the borers only attack more mature trees?
If you want to keep the tree, you will have to treat it or it will succumb to emerald ash borer. Here is a page about it, including how to decide whether it is treatable and by whom. (based on size) http://extension.umd.edu/hgic/invasives/emerald-ash-borer After this year, when the pesticide imidacloprid becomes unavailable to homeowners, you will need to hire a landscape company to treat for you each year.
Borers can attack at any time.
You will need to decide if it is worth trying to keep.