Today is my birthday. Let joy be unconfined. There won't be a party. Too stressful. The trouble with birthday parties, in my experience, is that you tend to group different friends into different pockets - you have work friends, and college friends, and various groups of random friends you've picked up along the way ... and since they're all quite different, you behave differently with them. I might be a swearing lout with one friend and an urbane sophisticate with another. Mix them all together in the same room and it gives me an identity crisis: suddenly I don't know who I am any more, and I panic and smash chairs against the wall until everyone goes home.

So instead of holding a birthday party, I plan to mark the occasion by screaming and crying. That's what I was doing the day I was born, so it's fitting. And besides, I've got cause for tears: apparently, I'm middle-aged. I'd always assumed middle age began somewhere in your 40s - the Oxford English Dictionary defines it as "the period between youth and old age, about 45 to 60" - but today's ruthlessly youth-oriented Reich has shifted the entry point ever closer, while I've grown steadily older to meet it. As I turn 37, I have to accept that I'm yesterday's news.

And just to underline how despicably aged I am, life has dealt me a small yet significant blow. For a while now, I've found that it hurts to type. Within moments of sitting at my keyboard, a headache-like sensation grows in my arm. The muscles creak. The elbow feels hollow. I'd always assumed that people with RSI were just making it up, the crybabies. Now I'm one of them.

So I've been seeing a physiotherapist. And, troubled by an apparent lack of progress on my part, she sent me for an MRI scan to see if there was anything going on in my neck.

Having an MRI scan is a barrel of laughs. First you sit in a waiting room, wondering why everyone else has a) come in pairs and b) looks so stressed. Then you realise they're probably waiting to find out about life-threatening tumours, while you're only there for an achy arm. This makes you feel a bit ashamed and unworthy, like someone simply having a go on the machine for a laugh. It also makes you contemplate your own mortality, or at least pull a face as though that's what you're doing.

Then you get changed, which simply means emptying your pockets and removing your belt, because although you don't have to be naked for an MRI scan, anything metal will make the machine spark, fizz, and explode, killing all known life forms in a 500-mile radius. Next you're led into a room occupied by a gigantic white machine with TOSHIBA printed on it. This is undeniably exciting, because you're going to lie down and go inside the big white tube and everything, like people who are ill on the telly do.

You lie down on a motorised tray. A chirpy assistant places rubberised ear-mufflers next to your head ("You'll hear a loud knocking sound in there"), then passes you a tube with a squeezy bulb thing on the end of it. If you start freaking out, squish it in your fist and they'll pull you out of the machine. "Ha!" you think. "Why would I freak out in the first place?"

And then you go inside the machine.

You glide inside surprisingly quickly, to find yourself staring upwards into a universe of featureless white. And then the noise starts. It didn't sound like knocking to me: more like an Aphex Twin gig. A series of stop-start electronic tones, buzzes, rumbles and alarms resonated through my head and neck. "This is what being a modem must be like," I thought, gazing into the bleached nothingness. It lasted about 20 minutes: more than enough time for anyone to start feeling seriously weird. Soon I became convinced I was having my mind wiped in a sci-fi thriller. Two minutes longer and I'd have been squeezing the freak-out teat and babbling about seeing through the Matrix.

On the way out, they give you a CD with your images on it - like a souvenir snapshot from a ride at Alton Towers, except instead of being depicted grinning on a log flume, you're dissected into slices. This is a bracing sight, and pretty good for kick-starting far deeper thoughts about your own mortality than the ones you were pretending to have earlier for the benefit of the people in the waiting room. As such, it makes perfect desktop wallpaper. Now, every time I minimise a window, I catch sight of my innards and contemplate death. This keeps me vibrant and alive and characteristically cheerful.

Anyway, the upshot of it all was, the scan revealed that my neck is older than I am. That is, a combination of bad posture, bad habits, and bad everything means part of my cervical spine appears prematurely clapped out. If I'm middle-aged, my neck's a pensioner. This is non-reversible, which means the resultant arm pain won't go away any time soon. Instead, I'll have to work round it, doing neck exercises like a codger. Age comes to us all, but I've managed to invite some of it to the party early, simply by not sitting correctly for 37 years. It's a humiliating birthday present, not to mention infuriating. I'd punch the wall in despair, except that would make things worse.

Perhaps I should hold a birthday party. After all, at my current rate of decay, next year's will be a wake, so I'd best make the most of each remaining moment. Just don't expect handwritten invites, OK?

This week. Charlie bought a popcorn machine: "It's my golden tip for anyone quitting smoking. Get one off Amazon for about 20 quid, and make bowl after bowl of warm, fresh, fat-free, air-popped popcorn. Ideal for tickling that oral-fixation cell in your brain each time you crave a cigarette, and shouldn't leave you fat as a house."