By synchronic wizardry, the two vehicles drive along - Smoothie Four- Wheel Dennis and Crazy Biker Dennis - side by side. The two men stop for lunch and sit at different tables, so that the sexy waitress can flirt with the older man (and ignore the hairy youth). Back on the road, they adjust their shades and, with a let's-get-serious gesture, the older Hopper roars off into the future leaving his earlier hippy incarnation far behind - stuck for ever in the slow lane, stranded in the past.
It's a work of genius, this 60-second comparison of bike and car, young and old, past and present, transient fashion and eternal cool, in which age wins out against youth. Mr Hopper is now 62. What a reassuring little dream for the mature male driver who still imagines himself kicking some butt on the highway. In terms of Shakespeare's "seven ages of man", it's the equivalent of the lean and slipper'd pantaloon carving up the lover and soldier at the traffic lights.
If only it were that simple - to reinvent the process of ageing as growing into coolness, as acquiring a kind of sexy wisdom with the advancing years. But it isn't really like that. Try as we may to halt the great wheel to which we are strapped, we know that the process of living is one of progressive degeneration. All the lifts and tucks and splints and medication and Viagra and Saga Venture Holidays won't conceal the process of irreversible decline.
It's especially tough on fortysomethings. The years of 40 to 45 are when parents die and children start hitting their teens and, by a weird form of mimesis, you start becoming your parents. Your attitudes harden along with your arteries. Your grip on both emotions and intellectual retrieval slackens along with your waistline. And your body starts to change, more drastically than at any time since the seismic hormonal disturbances of puberty.
A terrible lethargy has begun to settle on every joint in my limbs, as if a thousand under-used muscles were in terminal revolt. My hair, relentlessly greying since I was 30, is now snow-white, like King Lear's. I can no longer make out the road names in the index to the London A-Z, let alone the names on the maps themselves. Odd shooting pains invade my knees when I bend to plug in the standard lamp. My dentist looks aghast at my teeth, and announces that years of ferocious vertical brushing have pushed the gums virtually above my nose and down to my chin. If I move swiftly across the room to answer the telephone, the voice on the other end will say, "Goodness, you're out of breath - have you been running?". I've developed a curious habit of pressing an index finger against my temple while talking, as though giving my foggy thoughts a symbolic massage. If I stay up carousing past 4am these days, the bags under my eyes next morning are the size of steamer trunks; one day, they'll move into my face and stay there.
You can try, Hopper-like, to celebrate the liberation that comes with age. Look on this picture and on that, you say: the studenty prat in the sweaty ringlets at 19; the callow, smirking journalist, like a draper's assistant, in his first proper job at 28; the filled-out, languid fop at 40 in the Oscar Wilde hat; and you argue that evolution is heading the right way, shedding these foolish, unfocused identities like so many layers of dead skin. You can buy the theory that everyone has a perfect age. Some people are natural 18-year-olds. Some were always meant to be 26. Some greet 40 with a cry of recognition. Some are eternally 68. So it's some comfort to think that, though I was an unconvincing teenage groover, I've evolved into my natural identity as a clapped-out old roue.
There is one consideration, however, that stops you in your tracks. It's knowing that this is how you'll be until the end. Everyone at 50, declared George Orwell, has the face he deserves. And it's a face that, by and large, will remain that way until the accelerated collapse when you're 70 or 80, provided you aren't run over, or called in for "exploratory tests", next week.
My mother died this March, at 87. Until our last Christmas together, she was unchangingly herself, sharp-eyed, lean-faced, ginger-haired and tough, Dame Barbara Castle's younger sister. I once found her, aged 84, dragging a heavy dustbin through her front garden to await the refuse collectors. "For goodness sake, Ma," I said, "Let me do that. You're an old lady." Her eyes blazed. "Don't you dare call me such a thing," she snapped in real fury.
She went on busily living for 12 years after my father died, surfing the rolling years with infinite equilibrium. Time, when it caught up with her, took a terrible revenge. Accelerated by the cancer in her stomach, it laid waste to her face and body, covering her hands with liver-coloured blotches, making of her beautiful face a Monument Valley of sharp crevices and bluffs and secret folds, an unexpected new territory of stricken flesh. That was the final revelation of what Time is up to, what the ageing process is bringing us as well as, and instead of, wisdom.
She lived as a widow for 12 years in Oranmore, a small dormitory town near Galway. She was old and had a bowel complaint and was susceptible to loneliness, but she clung to her independence, and insisted on fending for herself in the house she and my father had bought. Such is the culture of Irish society, that she survived this reckless experiment.
The neighbours ran errands, brought her meals, kept her informed. The local nuns brought her Victoria sponge cakes and enquired about her wayward son in London. Friends, nephews and nieces, local priests and handymen all came a-calling, chatted to her, cut the grass, made cups of tea and kept her stocked with gossip.
Since she died, the whole subject of ageing and "ending up", far from being a ghastly, depressing thought, has started to fascinate me. Now, at 45, I'm aware of settling into an existential plateau, accepting that this income, this diet, this regimen of work and sleep and parties and reading and holidays and sex and Sunday lunch will spin through my days like a carousel with occasional minor variations. Everything I do, it seems, will become institutionalised, like pulling the Christmas tree lights out of the attic every year.
What will change will be the personnel. It's the knowledge that your children are so transient that afflicts you, the knowledge that your time with them can't last. That's when you start to consider how you'll end up. After a time, the possibilities resolve into two extremes. The negative impulses in your head tell you that, if you're lucky, your friends will die before you. That, if you're lucky, you'll just get immobile, haemorrhoidal, impotent, incontinent, deaf, bronchitic and barmy, rather than the bleak alternative. That, even if you're Dennis Hopper at 62, the chances are you'll have become Ronald Reagan 10 years later. That you'll sit, becalmed in a yellow-curtained day-room and a low-income bracket, unchanging, like a tree or the Queen Mother, while age and Time perform their final nasty tricks on you, making you die of boredom and loneliness.
More positive impulses tell you that all your friends will go on, like you, to 85, exchanging symptoms of minor ailments, and recommending hot toddies and holidays in Bali to each other. That you will always live in your own house, no matter what, with people calling once or twice a day. That you'll be able to drive, if only on B-roads, and will have enough money not to need insulting concessionary bus passes or cheap theatre tickets. That you will still be able to write and read and dance. That your eyesight will not fail you, or your liver or digestive tract, that your hips will not disintegrate, or your ability to talk sense and remember the words of songs. That your sense of humour won't pack up, shortly after your ability to taste and smell.
Old age is starting to land on more and more British men and women. The constituency of "older people" in this country is growing all the time. The figures are startling. There are 11 million pensioners in the UK at present; the figure will be 12 million by 2010. Falling birth rates and better health care mean that the classic pyramidal structure of abundant young people (at the base) and few oldsters (at the apex) is being turned on its head. The old will go on being old for longer. Half the population of over-75s have a long-term debilitating illness, but fewer people need regular medical attention; as a nation we're getting healthier.
The only thing that we aspirant wrinklies have to cling to is that, when we're old, there'll be an awful lot of us to lobby the government about larger pensions, flexible working hours, gradual semi-retirement and less age discrimination in the cardiac units of the NHS.
And when all the crucial issues of age have been tackled, by the government and by age charities, it is to each other that we'll have to turn for comfort. Some things are beyond money and laws, such as the Celtic generosity that filled my mother's last years with companionship and chat - in a word, society. When we've finished being Dennis Hoppers and celebrating our cool maturity, it will be time to start looking after each other. For it's in the way society cares for its oldest, most vulnerable citizens that its real value can be judged. As I shall write and inform Bob Dylan on his 60th birthday, two years from now.