I would say among the lower classes in America, it is the late teens to middle 20s. Poorer and less educated Americans have the habit of marrying very early and having children before they are educationally, financially, psychologically, and emotionally prepared. Also conservative and/or religious Americans tend to marry very early i.e. the teens to middle 20s at the latest. They have stricter, more narrow attitudes as far as lifestyle/relationships options and explorations go.
More affluent Americans, on the average, marry in their middle to late 20s at the earliest and by their 40s at the latest. More affluent and educated Americans believe in experiencing life and being prepared educationally, financially, psychologically, and emotionally before having children. They believe in bringing up their children in the best of circumstances and with the best of opportunities. Also more liberal and less religious Americans tend to marry later because of their more liberal attitudes to relationships and lifestyle exploration.