Image caption Barack Obama gets "Happy Birthday" sung to him at a Chicago fundraiser

Like most people, I picture President Obama celebrating his birthday in some "presidential" fashion - with a 21-gun salute and a parade, followed by drinks and cigars in the Rose Garden.

I never imagined he would celebrate turning 50 the way I did recently - by going a little crazy and running up my credit card debt by spending like there was no tomorrow.

And yet, President Obama has done pretty much the same thing (albeit on a much grander scale).

Age of irony

Perhaps the president's wrangling over the debt ceiling is really just a global expression of the familiar phenomenon brought on by this big birthday, where the prospect of turning 50 induces craziness, followed by wistfulness, accompanied by the decreasing ability to hold one's liquor.

Until the age of 50, you take your colon for granted

While turning 40 marks the true end of adolescence and for many people the "official" start of adulthood, turning 50 is the jagged milestone of middle life.

At 50, you seal your childhood in the distant past and know with certitude that the road ahead shortens, just as you scramble to meet it.

Fifty forces a person to cross the threshold into the age of irony.

By most markers, a 50-year-old is at the height of his power - professionally and personally.

Cover your roots?

But the wisdom gathered during earlier middle-age haunts this next phase of life - because at the very point when you feel you really know what you're doing, you start to notice the inexorable slip toward the margins.

Image caption His hair may be greyer, but at least he doesn't have a double chin

People start to tell you that you "look good for your age".

You notice the slightest little audible creak and groan as you rise from a chair. You troll the aisles of the pharmacy, examining hair colouring products promising to "cover your roots, naturally!" with the same attention you once paid to the products in the "family planning" aisle.

Most of your "firsts" are well behind you, except for that first recommended colonoscopy - which will be ahead of (and behind) you.

Suddenly, you become very aware of your colon.

Until the age of 50, you take your colon for granted.

You used to take your hairline for granted, and you'll never make that mistake again.

At 50, all of the movie stars seem to be younger than you are. You're older than professional athletes - and most of the coaches.

You start to worry about social coarseness, instead of gleefully contributing to it, as you once did.

At 50, other people really start to bother you.

Reason to celebrate

Unfortunately, at 50, you're also more aware of your limited ability to do much about other people.

Media playback is unsupported on your device Media caption President Obama enjoyed burgers on the eve of his 50th birthday

This includes your colleagues, friends and family - but especially your children.

Your kids can now out-run you, out-talk you, and humiliate you personally and on their Facebook pages.

You can only hope you've raised them well enough to pity you in your dotage.

This is the beginning of the grumbling, mumbling and whinging that will increase in frequency and volume through old age.

At 50, you pick up the cheque. Because at 50 the piper must be paid. And when the bill comes, everyone looks blankly at you while you wrestle with your wallet.

The best part of being 50 is the feeling that you've joined the ranks of the grownups.

The feeling that you know who you are and what you want.

You've matured beyond the stage of near constant amazement that anybody let you be in charge of anything.

Image caption The dad-in-chief said this week he was glad to hear his wife, Michelle, still thinks he's "cute"

If President Obama asked me for advice on this mid-life milestone (he told me once that he reads my column), I'd point out that at 50, when you start every day by seeing your parents stare back at you in the bathroom mirror, the real gift is that you're forced to reckon with the unavoidable, inescapable truth - that the luckiest of us will escape middle age into old age.

The idea is to do the least amount of damage while we get there.

And on that score, despite the historical challenges Barack Obama faces by being who he is on this day and in this time, I'd say he has reason to celebrate.