In theory, it's just a number.
But for many it's a crisis, signaling an inevitable, unavoidable look at the flip side.
For Michael Jordan, turning 50 carries greater implications. It means losing a battle with the calendar -- and this is not a man familiar with accepting defeat.
He was an immortal with a red cape, leaping skyscrapers in a single bound. Yet, suddenly, the mirror offers no solace. The only Magic he sees is when he runs into his old Lakers pal.
The greatest of all time -- the greatest of all the greatest who have ever engaged in anything resembling sport -- must finally come to terms with the fact that he will not return to the NBA.
If you understand Jordan, then you are certain of the moments in the last 10 years when he thought about it.
Seriously thought about it.
But he'll never again sniff his playing weight of 218, and we'll never again see Jordan pulverize a hand check and drive for a slam, screaming all the way to the rim at another imaginary insult created by his psyche's need to terminate.
And I fully admit to hoping for one more burst, just one more highlight.
Accepting that Jordan is 50 is really admitting that a wonderful part of my youth is also gone for good.
The memories, fortunately, do not fade.
There are a thousand, from the painful losses at the hands of Detroit, to the final, title-winning shot in Utah. In between, well, take your pick. You know them all as if they happened yesterday.
Mostly, when I think of Jordan, two amazing characteristics that defined him reverberate through my head.
First, is the way he reinvented himself whenever the Bulls had a need.
You became so familiar with the mind-boggling miracles, that the evolution was often hidden.
He came into the league playing above the rim and unable to shoot. Two years later he was deadly from the outside but with everything still hard off the dribble.
By 1990, he had become as a good a catch-and-shoot, clutch performer as anyone had seen, and a year later he was hitting John Paxson perfectly in stride, as the Lakers collapsed on Jordan -- and collapsed in the Finals -- as Jordan became the perfect, complete player.
When he retired in '93, he could get to the bucket and create any shot off the dribble or above defenders, but when he returned for his first full season in 1995-96, the Bulls had no post presence.
The Bulls were a team of specialists, and Jordan was forced to become their dominant post player. By elevating above defenders in the post, he created open shots off double teams and provided uncontested layups for the offensively challenged, while his fadeaway jumper down low was unstoppable.
In the finals months of the last title run, when his legs were tired and he couldn't consistently create a good look off the jumper with his back to the bucket, Jordan became a 6-foot-6 Kevin McHale and developed a floor move.
"I'm getting at my limit physically and at my limit mentally," Jordan said during the '98 Finals in Utah. "I'm maxing out on my basketball education. I'm not sure how much more there is to learn about the game."
But as simple as getting out of bed in the morning, Jordan flipped a switch, learned again and added a new weapon. Remarkable.
Mostly what I breathe in when I think of Jordan is his refusal to lose. What I exhale is his outright rage at the thought of coming in second -- at anything.
As long as there is a planet Earth, Jordan will be the definition of the need to win, and where it can take you in life.
He will always be the most competitive athlete who ever walked into an arena, the one who ruined franchise after franchise, and took such delight in doing so.
And we got to see every minute of it. Every spectacular minute of it. The greatest show of any kind. One that will never be duplicated.
I hope in the quiet of the remainder of his life that Jordan can find the peace and simple joy that has eluded him without an outlet to satisfy his need to destroy the opposition. That is, after all, the downside of being a self-trained assassin, that there is no replacement for the thrill of the kill.
The empty hole in his belly is a hunger Jordan can never properly feed again, and it is no easy task to manage his waking hours, pacing off the minutes, dreaming of a challenge that can mollify.
My daughter asked me the other day about Jordan. After 15 minutes of describing him, she chuckled at the unusual glee, pride and awe with which I spoke, items that don't often appear when I converse about sports.
I thought of ownership, too. Jordan was ours. He belonged to us. My goodness, how lucky we were.
So what one word -- I was asked -- would I use to describe that magnificent Michael Jordan era in Chicago?
Well, that's easy, I said.