“Middle age is a question not of math but of state of mind,” writes Steven Petrow. (Bryan Regan)

"I'm having a midlife crisis," I wistfully told my friend Amy earlier this year, shortly before my husband and I decided to divorce. I finally understood what W.B. Yeats meant when he wrote, "Things fall apart; the center cannot hold." Without even taking a breath, she replied: "You're too old. Unless you're planning to live to 120, you've aged out."

For the record, I was 59 (and she 56) at the time. But the bluntness of her retort took me down a rabbit hole of questions, the first of which was "When is midlife, exactly?"

A statistician would probably say midlife is the midpoint of our expected life span, which has grown by decades over the past century. My grandparents, born around 1900, faced a life expectancy of 49 years, according to the Centers for Disease Control and Prevention. By that statistician's definition, midlife began six months after their 24th birthdays. Jeez.

As for me, I was born in 1957, with a life expectancy of 70; my midlife came (and went) in 1992, when I turned 35. For a baby boomer who came of age not trusting anyone older than 30, this is particularly galling. The statistician's definition of midlife is clearly not for me.

Other experts have weighed in with other definitions of this thing we call middle age, most concurring with the American Heritage Medical Dictionary, which says it's "usually reckoned as the years between 40 and 60."

"What we think of as old has changed over time," said Sergei Scherbov, the lead researcher of a 2015 study that recalculated the math based on new data. "Someone who is 60 years old today," he said in a news release, "is middle-aged."

All of which is how I came to believe that middle age is a question not of math but of state of mind, as is its eponymous crisis. My impending divorce jump-started my crisis, this new search for meaning and purpose. Friends also told me that their midlife crises started after a disruptive milestone: Perhaps they became empty-nesters, or a parent died, or they became widowed.

Writers have much to say about midlife. "We suddenly or gradually find ourselves slowing down or unable to move our lives forward in familiar ways" is how Jett Psaris, author of "Hidden Blessings: Midlife Crisis as a Spiritual Awakening," explained the middle ages in a phone interview.

Ramona DeFelice Long, an author and editor who is 58, agreed: "From a woman's and parent's point of view, midlife is after children are grown and before retirement." That certainly doesn't peg it to any specific age these days.

Mark S. King, 56, a long-term survivor of the AIDS epidemic in Baltimore, piqued my curiosity when he told me that he felt his middle age had begun at 30, which is when he learned he was HIV-positive. This was the era when just about everyone who had AIDS died. "My life's timeline had been accelerated by the AIDS crisis. Everything compressed," he wrote in an email. "I faced the Big Questions in life then: What is it all about? What happens when we die? Is there a god?"

King's response hit home. I'd been in my mid-20s and a grad student when I was diagnosed with testicular cancer, which certainly provoked similar Big Questions for me.

Before my cancer diagnosis, I thought I was invulnerable, if not invincible. I was a smart-ass, a jerk, a smarty pants en route to a PhD who thought he knew so much about everything. Faced with a life-threatening illness, however, I became less certain. Wobbly. My previous identity melted away in the face of an uncertain future.

I don't think I entered middle age then, but my illness changed how I thought about my life's trajectory. It kicked off the beginning of what I'd call my "muddled years" — that time that novelist Judith Podell, 70, said is "like being in the middle of a large body of water and unable to see either shore."

That, it later dawned on me, is the central challenge of middle age, and of most midlife crises: reassessing your goals; redefining yourself without definition to others; finding an identity without a job or absent good health.

In the years after cancer, I found a new self. I was more vulnerable, less of a jerk, and increasingly humbled by how little I really knew. I could look back on my life while simultaneously looking forward, which seems the very definition of the middle. It was a hard redefinition of self at times.

I wish I could say that it was all smooth sailing once I got past those years, but it wasn't. My divorce, I realized, was a manifestation, and not the root cause, of another midlife disruption. After a dozen years together as partners, my sense of "me" had morphed into "we," and then my identity imploded as the "we" disintegrated. On top of that, my parents had recently died, and my role as a son no longer existed.

As Psaris explained: "The whole point of midlife is to allow the construct of who you are and the life you have created to fail. It's not just an opportunity for a fresh start; it's a mandate for one."

I've decided that, far from aging out of my midlife crisis, I'm going to embrace it. And my new mantra? Never trust anyone over 120.