The median age of marriage in the USA is older than as it ever has been - 26+ for women, 28+ for men. But the age of coupling - including cohabitation with children - seems to be about the same as it has always been - when you are done school and beginning adult life. When that is varies by class - about 19 or 20 for the less educated majority, and about 23 or 24 for the more educated minority.
Young people seem to be putting off marriage because they think that they need to be fully ready for adult life first. I think this is an impossible ideal. Moreover, it shows a misunderstanding of one of the foundations of a strong marriage: our commitment to each other, and to the institution of marriage, helps us become adults together.
At the same time, research on marital satisfaction shows that the happiest marriages tend to come from couples who marry in their mid-20s. And this is also the age that most young people and most parents of young people regard as the most desirable age for marriage.
Life leads to many compromises with our abstract ideals, of course, and most of the time these compromises work out.
Nonetheless, I am led to a radical idea for a social norm: plan to marry at 25 (give or take a year). This allows most people to make a good start at adult life, without setting an impossibly high ideal for being "ready."
Your mileage may vary, of course. But I believe that planning to marry at 25 would be a good norm for most people to plan around.