A new parent is perhaps most sensitive to the milestones of getting older. It’s almost all you talk about with other people, and you get actual metrics at the doctor’s every few months. But the milestones keep coming long after babycenter.com and the pediatrician quit with the reminders. It’s just that we stop keeping track.

Computers, however, have nothing better to do; keeping track is their only job. They don’t lose the scrapbook, or travel, or get drunk, or grow senile, or even blink. They just sit there and remember. The myriad phases of our lives, once gone but to memory and the occasional shoebox, are becoming permanent, and as daunting as that may be to everyone with a drunk selfie on Instagram, the opportunity for understanding, if handled carefully, is self-evident.

What I’ve just described, the wall and the long accumulation of a life, is what sociologists call longitudinal data—data from following the same people, over time. Even now, in certain situations, we can find an excellent proxy, a sort of flash-forward to the possibilities. We can take groups of people at different points in their lives, compare them, and get a rough draft of life’s arc.

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This approach won’t work with music tastes, for example, because music itself also evolves through time, so the analysis has no control. But there are fixed universals that can support it, and, in the OKCupid data I have, the nexus of beauty, sex, and age is one of them. Here the possibility already exists to mark milestones, as well as lay bare vanities and vulnerabilities that were perhaps till now just shades of truth.

As usual, the good stuff lies in the distance between thought and action, and I’ll show you how we find it. I’ll start with the opinions of women—all the trends below are true across my sexual data sets, but for specificity’s sake, I’ll use numbers from OkCupid. This table lists, for a woman, the age of men she finds most attractive. If I’ve arranged it unusually, you’ll see in a second why.

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Reading from the top, we see that 20-and-21-year-old women prefer 23-year-old guys; 22-year-old women like men who are 24, and so on down through the years to women at 50, who we see rate 46-year-olds the highest.

This isn’t survey data, this is data built from tens of millions of preferences expressed in the act of finding a date, and even from just following along the first few entries, the gist of the table is clear: a woman wants a guy to be roughly as old as she is. Pick an age in black under 40, and the number in red is always very close. The broad trend comes through better when I let lateral space reflect the progression of the values in red:

That dotted diagonal is the “age parity” line, where the male and female years would be equal. It’s not a canonical math thing, just something I overlaid as a guide for your eye.

Often there is an intrinsic geometry to a situation—it was the first science for a reason—and we’ll take advantage wherever possible. This particular line brings out two transitions, which coincide with big birthdays. The first pivot point is at thirty, where the trend of the red numbers—the ages of the men—crosses below the line, never to cross back. That’s the data’s way of saying that until 30, a woman prefers slightly older guys; afterward, she likes them slightly younger. Then at 40, the progression breaks free of the diagonal, going practically straight down for nine years. That is to say, a woman’s tastes appear to hit a wall. Or a man’s looks fall off a cliff, however you want to think about it. If we want to pick the point where a man’s sexual appeal has reached its limit, it’s there: 40.

The two perspectives (of the woman doing the rating and of the man being rated) are two halves of a whole. As a woman gets older, her standards evolve, and from the man’s side, the rough 1:1 movement of the red numbers versus the black implies that as he matures, the expectations of his female peers mature as well—practically year-for-year. He gets older, and their viewpoint accommodates him. The wrinkles, the nose hair, the renewed commitment to cargo shorts—these are all somehow satisfactory, or at least offset by other virtues. Compare this to the free fall of scores going the other way, from men to women.

This graph—and it’s practically not even a graph, just a table with a couple columns—makes a statement as stark as its own negative space. A woman’s at her best when she’s in her very early twenties. Period. And really my plot doesn’t show that strongly enough. The four highest-rated female ages are 20, 21, 22, and 23 for every group of guys but one. You can see the general pattern below, where I’ve overlaid shading for the top two quartiles (that is, top half) of ratings. I’ve also added some female ages as numbers in black on the bottom horizontal to help you navigate:

Again, the geometry speaks: the male pattern runs much deeper than just a preference for 20-year-olds. And after he hits thirty, the latter half of our age range (that is, women over 35) might as well not exist. Younger is better, and youngest is best of all, and if “over the hill” means the beginning of a person’s decline, a straight woman is over the hill as soon as she’s old enough to drink.

Of course, another way to put this focus on youth is that males’ expectations never grow up. A 50-year-old man’s idea of what’s hot is roughly the same as a college kid’s, at least with age as the variable under consideration—if anything, men in their twenties are more willing to date older women. That pocket of middling ratings in the upper right of the plot, that’s your “cougar” bait, basically. Hikers just out enjoying a nice day, then bam.

In a mathematical sense, a man’s age and his sexual aims are independent variables: the former changes while the latter never does. I call this Wooderson’s law, in honor of its most famous proponent, Matthew McConaughey’s character from Dazed and Confused.

Reprinted from Dataclysm: Love, Sex, Race, and Identity—What Our Online Lives Tell Us About Our Offline Selves. Copyright © 2015 by Christian Rudder. Published by Broadway Books, an imprint of The Crown Publishing Group, a division of Penguin Random House LLC.