My friend Ben is on the phone. He's just turned 26. "Twenty-six," he says, accenting the last digit, in case I miss the incredible significance. "It's a landmark birthday."

I begin to murmur agreement, the way I always do when someone states anything in a confident enough tone. But I'm already confused. To me, 26 doesn't seem like a particularly significant junction on the M4 of life. It's a landmark in the sense of the Esso garage on the corner being a useful landmark when you're giving directions to your flat, but it's not what you'd call a whopping great St Paul's Cathedral. "Surely 25 is the landmark?" I say. I've just turned 25 myself, and I don't like the idea of my own landmark being stolen from me. When you turn 25, you're nearer to 30 than you are to 20. That's serious.

Ben disagrees: "Because when you're 25," he says, "you're only halfway through your twenties. When you get to 26, that's when you're closer to 30. You can't be a teenager anymore."

Oh, for goodness sake. For as long as I've known him, every birthday has been The Big One: 19, final teenage year; 20, no longer a teenager; 21, officially an adult; 22, too old to be a member of Bis; 23, mid-20s; 24, mid-twenties (again). He makes them up as he goes along, devaluing the whole currency of landmarks in the process. How can he have a gigantic 40th birthday bash, if he has had nearly-40th birthday parties for the previous three years? He's like a greetings card company, dreaming up Granny's Day and Friend's Day and Gynaecologist's Day in an attempt to extort yet more cash out of us during the post-Valentine's lull.

I'm not fooled. There's something flimsy and unconvincing about every landmark birthday between 21 and 30. As a teenager, your birthdays are watersheds with actual legal significance: 16 is when you start wondering if everyone else has started having sex; 17 is when you can demand money for driving lessons; 18 is when you can refuse to vote. But what legal rights arrive after that? A twentysomething is allowed to do nothing new, except pay full fare on the train.

Still, I understand the landmarking urge. It's another subtle way of refusing to admit you're an adult. If you can cite 26 as an age when, for all intents and purposes, you've only just stopped being a teenager, then you can claim that the past few years don't really count. You can assert that there's another twist to go before you turn onto that clear, straight, scary road that leads to death or middle-age, whichever seems grimmer at the time. In theory, you can put off your thirties until it's time for you to retire. Oh, and if you call your birthday a landmark, it's also a sly method of announcing that you expect an extra-special present.

So, I can't blame Ben for throwing yet another obstacle in the way of adulthood, but I can't approve of it, either. Twenty-one is a landmark. I'll give you 25, at a pinch. But the next one isn't until you turn 30, and that's a different crisis altogether.