The age of marriage is going up, so the proportion of people who are married in their early 20s - the traditional age of marriage - is going down.
However, the cohabitation rate is going up, and it starts earlier than the marriage age. Indeed, most couples cohabit before marriage. Some do so as test for marriage, though that does not work as well as they hoped.
The upshot is that Americans couple up, move in together, and even start to have kids at about the time and rate that they always have.
As the new CDC report, First Marriages in the United States puts it,
"If entry into any type of union, marriage or cohabitation, is taken into account, then the timing of a first union occurs at roughly the same point in the life course as marriage did in the past."