Birth rates for U.S. teens and early 20-something are at (another!) all-time low, and birth rates continue to rise at ages 30 and older.
As a historical demographer, who has some experience with fertility and mortality rate trends over the past century (and the century before), and I can say with conviction that the trend toward higher birth rates at ages 30+ is not really new. Birth rates for women ages 35 and older are not higher now than ever. (They're not even higher now than they were in the 1950s and 1960s.) I would argue that, rising birth rates among those in their 30s and 40s is more a return to long-run historical norms than an aberration. (First births at older ages is a newer phenomenon, the rate at older ages is nothing new.)
The National Vital Statistics Reports, published by the U.S. Centers for Disease Control and Prevention, provide historical birth rate data by age of mother as far back as 1970. Earlier years are available, but must be compiled from a variety of other sources including the older, and often PDF-scan-only Monthly Vital Statistics Reports and the U.S. Statistical Abstract. From those sources I collected data as far back as 1920, with complete annual data from 1935-present. The historical birth rates (births per 1,000 women) are shown in the chart above.
I am happy to share the raw data upon request. Feel free to contact me for more information.