In order to be considered "old growth," a forest must no have been logged for over two hundred years. Such forests can contain trees that are hundreds, and in some instances even thousands, of years old. Old growth forests aren't just novelties; they also unique ecosystems and windows into the past.
The key to this helpful (and profoundly sad) graphic explains that each dot represents 25,000 acres of forest. There are smaller stands of old growth throughout most of the country; this Wikipedia page lists the location of many pockets of old growth. Something worth checking out before it's gone.