The last time the marriage rate declined was in the 1930s, and the combination of economic and government-related (e.g., taxation) issues were similar then as they are now. My impression, moreover—it would be interesting to see if this is statistically the case—is that the rise in marriages and births that took place after WWII was to men who fought in the war, meaning they would have been too young to be married in the 1930s, married to wives whose male counterparts who were too young to have fought in the war. If this is the case and if history is to repeat itself, then the marriage rate should rise beginning around 2020, not because the older millennials will be marrying, but because the younger millennial men will be marrying post-millennial women.
Hard to get married when your living in your parents basement looking for a job.
I may not recall this correctly, but I remember from my medieval European history class that there were similar delays in getting married based on bad economic factors, and earlier marriage in good economic times at that time as well.