It is no uncommon when examining the doctrinal statements of fundamentalist churches that both dispensational pre-trib premillennialism (Left-Behind-ism) and Young Earth Creationism are both elevated to essentials.
Dispensationalism owes its popularity to C. I. Scofield who was a genius. A misguided genius, but nevertheless a genius. Scofield did something that was then novel. He published his Scofield Reference Bible (1909, rev. 1917) in which he embedded his notes and extensive cross-referencing scheme--unambiguously written from a dispensational viewpoint, into the biblical text rather than in a separate commentary. Combined with the fact that his notes were written with an air of absolute authority left many believers with the impression that Scofield's commentary had been vetted by ages and sages.
So, back to the fundy churchs that demand fealty to both dispensationalism and YEC-ism.
What about the hero of dispensationalism, the undisputed heavyweight champion, C. I. Scofield? He needn't apply. Maybe the Methodists will take him. Why?
Because C. I. Scofield was an Old Earth Creationist.
It is interesting--dispensationalism is the only systematic theology developed in the scientific era. As such, Scofield was well aware of fact that geology teaches us that the earth is old. So he embedded a particular form of OEC into his notes: the gap theory. He taught of an unknowable (from scripture, at least) long period of time between the first verse of the bible and the second. When he picks it up in the second verse he sounds like a YEC--he taught literal 24-hour days and even included Bishop Usher's calculations (with the dreaded 4004 BC result) in his original notes. So many people think was a YEC. But he wasn't.
Even in the 1967 New Scofield Reference Bible (from which the Ussher chronology was purged) the notes in Genesis state that the age of the universe is unknowable from scripture.