From the time they could state their ages correctly, my kids have wanted to know how old I am.
While they could grasp the age thing - you add one every time it's your birthday - they struggled with the larger concept of time.
How often do you celebrate a birthday? Once a year? How long is that?
When they were younger, they would ask me my age every few months or weeks.
Then, inevitably: "Are you old, mama?"
"Older than you, but I'm not old," I would say blithely.
I meant it too - until 2015 dawned. Suddenly, I grew acutely aware of my mortality. Even my kids could sense this was no ordinary milestone.
"You are going to be 40 this year," my son, now eight, exclaimed a few months ago.
"That's old, right?" his five-year-old sister asked in a mix of wonder and worry.
The usual denial stuck in my throat. While I would hate to think so, I'm afraid the signs are not encouraging.
Grey strands have started sprouting at an alarming rate. Sore muscles, post-exercise, stay sore for longer. And while this should come as no surprise and changes nothing, it recently hit me that I'm probably older than all my kids' teachers.
Yup, I'm on the cusp of middle age - or old age, going by the lexicon of certain authorities.
In April, three months before my birthday, I received a thick envelope informing me that I'm now eligible for ElderShield, a disability insurance scheme for long-term care of the elderly.
I was annoyed. "This ElderShield thing needs serious rebranding," I griped to friends who had felt similarly slighted when the dreaded package landed in their mailboxes.
Another rude jolt awaited me.
My personal trainer, an affable chap in his early 20s, seemed shocked when he learnt I was turning 40.
"You don't look it," he said, then promptly ruined the compliment by asking in all earnestness: "Were there already HDB flats when you were born?"
So this is what being on the wrong side of 40 is like.
While your desirability index plunges, your value as a historical resource and novelty appreciates.
Increasingly, I find myself adding the "when I was young" prefix to conversations with my kids. It's a good thing they are still young enough to not roll their eyes at me and even enjoy some of these stories.
I now know why a senior editor used to bristle at the thoughtless use of the word "still" in our copy. This actress was "still hot at 40", we might write. Or that mother of three was "still fit at 50", say.
Then in our invincible 20s, we shrugged off his indignant messages as over-reaction. We had meant it as a compliment, couldn't he see? Finally, I get why the innocuous word can be so insulting.
No one wants to be written off and definitely not at 40.
A few years ago, USA Today ran an article hailing 40 as "the new Sweet 16", as many Gen-X women are marking the milestone with parties and vacations.
"But rather than a coming-of-age statement, the 40th party is a way to proclaim they're healthy, they're sexy and they haven't lost their mojo," it reported.
As economist John Shoven told the newspaper, 40 no longer means what it did a generation or two ago.
Then, women seemed destined to fade into oblivion once they raised a brood.
"When a woman turns 40, she is not the same age as when her mother turned 40. She's in better health," Professor Shoven noted. "She has a lower mortality risk. She has more healthy years to look forward to."
And so it is. The fifth decade is when today's mums start reclaiming me-time with a vengeance.
Looking back, it seems as though I've hurtled through my 30s largely in a haze of stress and fatigue.
I had two kids in three years while trying to make my mark in my career and I think I ended up excelling in neither role. I was sleep-deprived for at least half of the decade and sedentary for most of it.
But as my kids grow older and are no longer mere appendages of me, I'm relishing having my life back. By carving out more time for myself, I'm actually enjoying them more.
In the last six months, my husband and I have gone on two kid-free weekend getaways with a bunch of friends and are planning a third such vacation.
Resolving to brave the second half of my life in optimal health, I stepped up my exercise regimen earlier this year. I started weekly tennis lessons with a friend, signed up for pilates classes with two others and try to hit the gym twice a week.
With my birthday a month away, I don't think I've been in better shape since my school days. Forty and fabulous? Not quite, but I sure want to give it a shot.
I know I will never regain a flat stomach or plump skin, but I've accepted that I'm no match for what has been summed up as the ravages of time, gravity and childbirth.
I'm no longer plagued by the neuroses of a 20something, but am driven simply by the wish to stay in good health so that I can enjoy myself and my kids.
The next time they ask if I'm old, I will quote what general practitioner Ciara Kelly wrote in the Sunday Independent last year of women in their 40s: They are too old to be self-conscious and too young to be past their best.
This means I'm not going to stop bopping to my favourite songs on the radio even when my kids tell me the driver in the car next to ours at the traffic light is staring at me. This means I can still out-run and out-swim them for at least another five years.
I think we are all set to enjoy my 40s together.