When it comes time to choose between working it out and breaking up, the folks who got married later have much more practice at breaking up than staying together, so it seems like an easier thing to do.
Bingo. I strongly suspect that it is the number and depth of previous relationships (break-ups) that is the real predictor of eventual divorce and not simply marrying older. Although people who wed later tend to have had more serious break-ups than those who marry earlier, the number of serious break-ups is almost assuredly the causative factor. It will take a differential study to definitively identify the true independent variable at play here, but it seems obvious to me even though I married in my mid-20's.