Unless you're reading this later in the week, in which case I was 21 on Thursday and haven't received your card yet (or reading this at the dentist, in which case I'm happily married and living in a bungalow in Finchley).
I realises it's risky to admit my age just in case the Herald powers-that-be realise I've lost my status as young person and oust me for a spritely 15-year-old.
But in the spirit of the occasion, I'm going to be reckless today. Heck, I might even have crisps for breakfast.
I'm realising, as my youth withers away before me like an unattended pot plant, that there were lots of things I always meant to do before I reached 21*.
As a wide-eyed teenager, I thought of 21 as the age of Genuine Actual Adulthood Horror (or GAAH for short).
For starters, it's the last birthday before 50 where people buy you really good presents.
From 22 onwards, as I've been reliably informed, it's nothing but bath salts, book tokens and back pillows.
Worse, I sense I've reached the age where I can no longer blame my incompetence on the giddy innocence of youth.
Next time I fix a broken laptop charger with masking tape, which subsequently catches fire and fills my room with noxious glue fumes, I don't think people will think it's an endearing foible. I think they'll think I'm an imbecile.
I thought I would be a better-informed person at 21.
I thought that by now, I'd know the difference between a bank and a building society.
I thought I'd be able to drink Lemsip without needing a biscuit to take the taste away.
I'm ashamed that I still, after five years of going "yeah, must upgrade that" every time it's produced in restaurants, have a Solo card instead of a proper debit card for grown-ups.
And after years of quiet wondering, and the feeling that I really ought to have found out by now, I still don't quite understand what Che Guevara did.
A 21-year-old should know these things.
I should be able to buy a lightbulb without having to call home to check once more whether we're bayonet or screw-in.
I should be able to use all the settings on our washing machine as appropriate, instead of always using 40 and just hoping for the best.
And I should definitely have stopped ironing everything with my hair straighteners.
Still, if I'm lacking in some of the fundamentals of successful adulthood, I guess it's a comfort that I'm also avoiding those depressing symptoms designed to keep grown-ups distracted from their mortgages and stress incontinence.
In 21 years of happy dress-wearing, the purpose of a slip still evades me. I think I can probably stave off slippers for another 10 years.
And I still feel confident that at no time in the near or distant future will I ever feel the pressing desire to purchase an Airwick plug-in.
Perhaps soon I'll hear the call '” I'll be strolling past a display of those TV dinner trays with bean bags underneath and instead of shuddering I'll think "My, what a practical solution to a common dining conundrum."
Then I can rush off to open and file all my bank statements, register with a dentist and buy a new laptop charger.
Until then I plan to make 21 the new "it" age '“ and it's starting with Monster Munch.
* One of them was watering the pot-plant.
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