Next week, my wife reaches a milestone; not one with a round number, but perhaps more significant: She fills 75 percent of the century-long container she was born into. She has passed her “three score years and ten” by five.

While she is hampered by a dozen ailments requiring enough medicine to count each morning as a full breakfast, none is immediately life threatening.

But it started me thinking about those traditional milestones we set for ourselves. There are annual birthdays that we count off, but really, is there that much difference between being 32 and being 33? So, there are longer stretches that actually count out time that feels significant. For many, watching the odometer turn over from 29 to 30, or from 39 to 40 is accompanied by an unwelcome breakfast of existential angst.

“Am I really getting old?”

Gail Sheehy wrote a famous book about the changes we go through as we grow. But Passages is more about the psychology of such changes. What I’m talking about are the arbitrary milestones. In Sheehy’s book, the lifestages are sequent, but not hardwired to a specific age, but a likely decade — your 20s or your 30s. What I am looking at here are not the stages themselves, but the signposts that we recognize as they pass.

For me, this starts with age five, when we first leave home daily to attend kindergarten. It is a great wrench in our lives, and we are no longer always safe in our nests, cared and fed by an attentive mom.

The next big one is when we turn thirteen. Eleven and twelve are technically “teens,” but only when we hit thirteen does it seem to count, perhaps because the syllable “teen” is explicit. On your thirteenth birthday, you proudly consider yourself no longer a child, but an adult, or at least and adult manqué. “Today, I am a man,” goes the rite of the bar mitzvah, along with the gift of a fountain pen.

Next up is a bifurcation of genders. Girls have their “sweet sixteen” party, or, if Latina, their quinceañera, marking their fifteenth or sixteenth birthday. In the past, this advertised their marketability as brides, although nowadays, when people marry later, it is a vestigial celebration of “coming out” as a wearer of party dresses.

For boys, sixteen goes by unannounced. When I was growing up, boys had their counterpart at 18, when they signed up for their Selective Service card, marking their eligibility for the military draft. Then, it was accompanied by a deep hard swallow and a nervous smile waiting for a letter from the draft board. Nowadays, without an actual draft, registration is largely a formality.

For boys and girls, there is the age of the learner’s permit for driving, but this varies widely from state to state, beginning usually at fifteen, but for some not till eighteen. Northern states tend to a later age, Southern states tend to let their bairn get behind the wheel much younger.

Eighteen also marks the official “age of consent,” demarking the legality of sex. This is currently an age more honored in the breech than in the observance. It is also voting age, also honored more in the breech.

For most states, eighteen is also the age of legal majority, save Mississippi, where you still must reach twenty-one.

Twenty-one used to be the standard bar for majority, when you become legally responsible. In less enlightened ages it was accompanied by the boast of being “free, white and twenty-one.” Now it is primarily the age at which it becomes legal to get drunk. It is the signal irony that just at the moment the person in question finally declares him- or herself a grownup, he (or she) is most likely to do the most immature thing imaginable.

Most of these age signposts seem front-loaded into growing up. After that, they become more spaced out over time. For some, turning thirty is a hazard, for others, it hits turning forty; it is the moment you realize you will never be young again, that the responsibility of adulthood, family, career and citizenship have replaced dating, playing and experimental sex. Sobriety hits, and it isn’t always fun.

Fifty is a quiet marker; most people find they are relatively happy at that age. There is, perhaps, more satisfaction at having reached the half-century mark than unease at getting on in years. Most of us, at fifty, are still vital and energetic, and we have the added benefit of all that accrued experience.

(When I was young, I calculated how old I would be at the turn of the millennium and realized I would be fifty-two, and I was daunted because I really didn’t believe it likely I would live that long. When it actually happened, it almost snuck up on me. I was hardly aware I was over fifty.)

Sixty is stealthy, because it comes and goes hardly noticed because one’s eyes are firmly on the big sign ahead: sixty-five and the finality of retirement. They keep moving the finish line on us, upping the age for Social Security, but sixty-five is so deeply entrenched in our collective psyches, that the subsequent years just seem like a tiny hesitation. 

The round numbers seem less meaningful than the fivers. Seventy-five is three quarters of a century and an accomplishment more than merely a signpost. Beyond that, the years seem less important than the life put into them. Currently the average life expectancy for American men is seventy-seven and for women, just under eighty-two. Past that, the only one that really counts is the even hundred. The George Burns point. Beyond that, you begin to count every years once again, just as you did as an infant.