In truth, the extremely low first marriage ages of the post-war years were an aberration. Contrary to popular myth in the "old days" men didn't get married at all that young of an age and women didn't all get married on their 18th birthday. It's in the 1950-1970 era that the marriage age shrunk radically, with men and women marrying at more similar ages.
The key difference now is that both male and female first marriage ages are going up in tandem. The age gap between the genders shrunk in the 50-70 years as both men and women were marrying earlier, and then the gap remained relatively small as members of both genders began to delay marriage until a later time.