I am about to go to Puerto Rico for a week, to work on a book and be looked after by my 83-year-old friend John, one of my two best octogenarian friends. I am close to perhaps half a dozen septuagenarians and couldn't begin to count the number of sexagenarians who matter to me. Even the tenant of my affections is no spring chicken.

I'm not ageist. I even like some teenagers. But others are, and eyebrows are raised at my relationships with people substantially older than me. There is still a widespread presumption among the undiscerning young and middle-aged that the aged are nuisances and killjoys. Yet I find them among the most amusing, life-enhancing and easiest of all my companions. People who were interesting and fun at 50 will be the same at 70, and have good reason to get the maximum pleasure out of each day. And most of them will have the time to enjoy themselves and nurture their friendships.

I've acquired several elderly friends through my work as a biographer and historian, for, again, it is the old who have the time to give a lot of help and encouragement. And precisely because I am curious about their lives and the times through which they have lived, we have much to talk about: I'm as interested in hearing insider gossip on Lloyd George or Selwyn Lloyd as on Michael Portillo.

I've become used to the luxury of staying with people whose brains I am picking, whose memories I am jogging and whose personal papers I am reading, and being nurtured all the while like a cherished invalid. People who 20 years ago were working 16-hour days feed me delicious meals, chat over the wine (no nonsense about designer water) and encourage siestas or evenings off watching old movies. Some of them decide that their main job when I'm with them is to ensure that I go home rested rather than even more knackered than when I arrive. I have a lovely time, we enjoy each other's company, and so of course some of us become real friends.

I acquired the 83-year-old ex-journalist John by that route years ago. "I'm too fucking old," he says, when asks how he is, but until he got some back trouble last year he zoomed around the world regularly to see his friends. At home in Washington or in various holiday places, John now sees his main role as being to look after his much younger wife and their endless stream of house guests of all ages. He resembles another lovable journalist, Brian Inglis, who in his seventies used to entertain more regularly than anyone I know. When he died at 76, he left behind a hard core of bereft, much younger people, who used to lunch with Brian most Sundays but are now almost always too busy to cater for each other.

It isn't just because they have time that the old frequently are better at being friends than they were in youth or middle-age. it's also because they have perspective and wisdom. They may be irritated by physical troubles, but they are mostly untroubled by angst. You don't have to sit around with an 80-year-old talking about his emotional crises: you can sit around telling him about yours. Most of the problems of age are practical. You don't find many old people rushing off to shrinks.

If you find wisdom an attractive characteristic, what is on offer from old people with lebenslust is almost always palatable. They have lived long enough to know there is no point in trying to change others, so they confine themselves to reflections of the sort at which Ernest Bramah's character, Kai Lung, excelled. ("It has been said that there are few situations in life that cannot be honourably settled, and without loss of time, either by suicide, a bag of gold, or by thrusting a despised antagonist over the edge of a precipice upon a dark night.")

Work? It was John who taught me the invaluable phrase, "Don't get it right. Get it written." Money problems? My septuagenarian friend, Sean, recommends taking public transport so that one can always afford champagne for one's friends. (The tenant of my affections, on the other hand, thinks life too short for public transport and recommends instead working faster and demanding twice as much for it.) Enemies? The octogenarian Hugh is in favour of inviting them to a club to which we both belong and giving no warning that immediately behind the front door is a lethally steep staircase.

I suppose I acquired a taste for the old partly because of my parental septuagenarian anarchy. My father rather overdid his attempts to behave like an 18-year old, but there were a lot of laughs along the way. My mother specialised in hoodwinking the innocent by her poor-old-lady routine, while staying up talking till 5am any time the opportunity presented itself. I will not easily forget the occasion when at the age of 77 or so she rang me and my then husband from Dublin at 11am to say she was on the point of death through grief at my father's latest misdemeanours. At 11.15am, tremulously, she agreed to drag herself to London to die peacefully with us. At 5.30 pm, she emerged from Heathrow customs in a wheelchair beaming. At 6pm, she said of course she was fit to go out to dinner with us and at midnight she was performing the sword dance with our Scottish host.

It is because of my parents that I realised quite some time ago that just because you're old, it doesn't mean you are dull.