It started with soft percussion ("Bap bap b'bap-bap, bap bap b'bap-bap") into which there suddenly fizzed the slow glissando of a G chord played upside-down on a 12-string guitar ("Zzzzoooinnnggg") and that flat English voice came in: "Pushing through the market square. So many mothers sighing. News had just come over – we had five years left to cry in..." It was so cool, the announcement of the end of the world. And it announced that something else had arrived: a new musical genius for the Seventies generation, a new look and sound to inspire us.

"Five Years" was the opening track on The Rise and Fall of Ziggy Stardust and the Spiders from Mars, the classic album by David Bowie released on 6 June 1972. Its 40th birthday year was celebrated this week by the unveiling of a grey plaque in Heddon Street, London, where the cover was shot by Brian Ward in January 1972. By October that year, when I went to my first term at university, the record had become my emotional crutch, and the protean figure of Ziggy my alter ego.

The fictional Ziggy is an alien "starman" who falls to earth, learns the guitar, forms a band, becomes the last rock star on a dying earth, has bisexual frolics as Ziggy/Lady Stardust, drinks beer and is abused by fans and band colleagues until, deciding he's lived too long, he commits suicide. That's a lot to try and empathise with, but it didn't matter.

Some of the songs didn't have much to do with the alien, and that didn't matter. Philistine friends said the music was derivative, that "Starman" was, basically, Judy Garland's "Somewhere Over the Rainbow," while "Hang Onto Yourself" and "Suffragette City" were 60 per cent Eddie Cochran, but even that didn't matter. What mattered was the passion of pure fandom.

I identified with every note of Mick Ronson's honkingly metallic guitar sound, with every weird cockney-quaver of Bowie's voice. I learned to play "Five Years" and "Starman" on guitar. To prepare for a talent night in the college bar, I took a girlfriend to the Oxford shops to find an outfit. In Miss Selfridge, we bought a zip-fronted, black lamé jumpsuit covered in gold stars (frankly, the top would have looked better worn by someone with breasts) and a pair of suede boots, which I spray-painted silver and dotted with WH Smith gold merit stars.

I was a man possessed. I looked a fright and didn't care. I went to fancy-dress parties as Ziggy in hideous outfits – the silver-grey leotard was a low point – until one evening I heard someone say, "Who's that who's come as Widow Twankey?" Suddenly, the jig was up.

I finally saw Bowie live on stage a year later. He was great but he'd moved on to become Aladdin Sane. The audience was full of stupid youths with blue-red flashes on their faces, trying to look like him. (Imagine.) So I moved on too. Years later, I'm still pleased to have once found myself in the bliss of total-immersion fandom. It's like falling in love or living under a dictatorship in which everything the beloved leader does or says seems the actions or speeches of a genius. It makes no sense, but it means everything.