Over the past half century, parenthood has undergone a change so simple yet so profound we are only beginning to grasp the enormity of its implications. It is that we have our children much later than we used to. This has come to seem perfectly unremarkable; indeed, we take note of it only when celebrities push it to extremes—when Tony Randall has his first child at 77; Larry King, his fifth child by his seventh wife at 66; Elizabeth Edwards, her last child at 50. This new gerontological voyeurism—I think of it as doddering-parent porn—was at its maximally gratifying in 2008, when, in almost simultaneous and near-Biblical acts of belated fertility, two 70-year-old women in India gave birth, thanks to donor eggs and disturbingly enthusiastic doctors. One woman’s husband was 72; the other’s was 77.

These, though, are the headlines. The real story is less titillating, but it tells us a great deal more about how we’ll be living in the coming years: what our families and our workforce will look like, how healthy we’ll be, and also—not to be too eugenicist about it—the future well-being of the human race.

That women become mothers later than they used to will surprise no one. All you have to do is study the faces of the women pushing baby strollers, especially on the streets of coastal cities or their suburban counterparts. American first-time mothers have aged about four years since 1970—as of 2010, they were 25.4 as opposed to 21.5. That average, of course, obscures a lot of regional, ethnic, and educational variation. The average new mother from Massachusetts, for instance, was 28; the Mississippian was 22.9. The Asian American first-time mother was 29.1; the African American 23.1. A college-educated woman had a better than one-in-three chance of having her first child at 30 or older; the odds that a woman with less education would wait that long were no better than one in ten.

It badly misstates the phenomenon to associate it only with women: Fathers have been getting older at the same rate as mothers. First-time fathers have been about three years older than first-time mothers for several decades, and they still are. The average American man is between 27 and 28 when he becomes a father. Meanwhile, as the U.S. birth rate slumps due to the recession, only men and women over 40 have kept having more babies than they did in the past.

In short, the growth spurt in American parenthood is not among rich septuagenarians or famous political wives approaching or past menopause, but among roughly middle-aged couples with moderate age gaps between them, like my husband and me. OK, I’ll admit it. We’re on the outer edge of the demographic bulge. My husband was in his mid-forties and I was 37—two years past the age when doctors start scribbling AMA, Advanced Maternal Age, on the charts of mothers-to-be—before we called a fertility doctor. The doctor called back and told us to wait a few more months. We waited, then went in. The office occupied a brownstone basement just off the tonier stretches of New York’s Madison Avenue, though its tan, sleek sofas held a large proportion of Orthodox Jewish women likely to have come from another borough. The doctor, oddly, had a collection of brightly colored porcelain dwarves on the shelf behind his desk. I thought he put them there to let you know that he had a sense of humor about the whole fertility racket.