By the first half of the 19th century, educated Christians accepted that the geological evidence comprehensively ruled out a 6000 year old Earth. As long ago as 1848, John Thomas casually alluded to an Earth millions of ages old. Robert Roberts, the first editor of The Christadelphian likewise accepted the overwhelming evidence for an ancient Earth, while his successor C.C. Walker flatly stated back in 1911 that:
"[t]en years ago the average scientist would have asserted that our habitable globe had not existed for more than a hundred million years. Now it would be hard to find a competent physical specialist who would fix a definite maximum below a thousand million years:’ [1]
Advances in geophysics have allowed us to fix the age of the Earth to around 4600 million years, a figure that is around six orders of magnitude greater than the YEC figure. If Walker was alive today, he would have no reason to recant his statement that it would be difficult to find a 'competent physical specialist' who would endorse a young Earth. The evidence is beyond dispute.
The evidence is not just from radiometric dating, powerful though that line of evidence may be. As geoscientist Jonathan Baker points out, the evidence for an ancient Earth comes from areas as diverse as dendrochronology, coral banding, glacial cycles, stromatolite formation and the cooling times for igneous intrusions.
What this means of course is that if Christadelphian YECs can't even get right the most basic facts about geology, there is no reason why we should pay any attention to their other claims about how God created all living things.
1. Walker. C.C, ‘The Age of the Earth’, The Christadelphian (1911) 48:450
