Indeed, modern neuroscience and psychology confirm what Aristotle said more than 2,000 years ago when he described men in their “primes” as having “neither that excess of confidence which amounts to rashness, nor too much timidity, but the right amount of each. They neither trust everybody nor distrust everybody, but judge people correctly.”

I agree. We’ve actually managed to learn and grow a bit. We see the hidden costs of things. Our parents have stopped trying to change us. We can tell when something is ridiculous. And other minds are finally less opaque. The seminal journey of the 40s is from “everyone hates me” to “they don’t really care.”

Even so, the decade is confusing. We can finally decode interpersonal dynamics, but we can’t remember a two-digit number. We’re at or approaching our lifetime peak in earnings, but Botox now seems like a reasonable idea. We’re reaching the height of our careers, but we can now see how they will probably end.

And this new age is strangely lacking in milestones. Childhood and adolescence are nothing but milestones: You grow taller, advance to new grades, and get your period, your driver’s license and your diploma. Then in your 20s and 30s you romance potential partners, find jobs and learn to support yourself. There may be promotions, babies and weddings. The pings of adrenaline from all these carry you forward and reassure you that you’re building an adult life.

In the 40s, we might still acquire degrees, jobs, homes and spouses, but these elicit less wonder now. The mentors and parents who used to rejoice in our achievements are preoccupied with their own declines. If we have kids, we’re supposed to marvel at their milestones. A journalist I know lamented that he’d never again be a prodigy at anything. (Someone younger than both of us had just been nominated to the United States Supreme Court.)

“Even five years ago, people I met would be like, ‘Wow, you’re the boss?’” the 44-year-old head of a TV production company tells me. Now they’re matter-of-fact about his title. “I’ve aged out of wunderkind,” he says.

What have we aged into? We’re still capable of action, change and 10K races. But there’s a new immediacy to the 40s — and an awareness of death — that didn’t exist before. Our possibilities feel more finite. All choices now plainly exclude others. It’s pointless to keep pretending to be what we’re not. At 40, we’re no longer preparing for an imagined future life. Our real lives are, indisputably, happening right now. We’ve arrived at what Immanuel Kant called the “Ding an sich” — the thing itself.