Jana Riess conveys finding from the Next Mormons Survey that indicates that median age for marriage among Mormons is 22.
Jana Riess, "10 things to know about Millennial Mormons," Flunking Sainthood, May 24, 2017, accessed November 16, 2021
One thing to note is that despite some hand-wringing in the Church about the perils of delaying marriage (see a recent article about this in the Ensign), there’s no evidence that either current or former Mormons are in fact delaying marriage–of those who get married, the median age for current Mormons to do so is 22, and for former Mormons it’s 23. They are, however, having smaller families.