I had my first child when I was 38 and my second — my 15-month-old daughter — at 41. Though I am, by most people’s criteria, an older mother, like many of my friends who also had their children in their late thirties or forties I don’t feel remotely old. I still wear Topshop, race around on the beach with my kids, buy inappropriate shoes and read Heat magazine. Yes, when planning to have our children most of us worried a little about declining fertility, antenatal testing and feeling knackered. What didn’t cross our minds was the impact that this would have on our becoming grandparents.

But when my daughter was born I suddenly realised that if my two waited as long as I did to have children, I would be at least 79 before I became a grandmother. And 79 is old.

Of course there are plenty of 80-year-olds who are fit as a fiddle. But what if I become old as in frail, senile or even dead? Many of my contemporaries confessed to similar feelings of shock at realising how the numbers stacked up.

So is this the future? Ancient grannies who can only watch as their grandchildren grow? Or do advances in healthcare and a constantly increasing lifespan mean that we can look forward to a generation of sprightly supergrannies who stay actively involved well into their eighties? The age for first-time parenthood is rising all the time, particularly among the middle classes, where education and career tend to take priority over finding a mate or making babies. In the past few years the average age of first-time mothers has risen to almost 29.

For the first time, fertility rates of women in their thirties are outstripping those of women in their twenties, and the number of fortysomething mothers, such as Cherie Blair, Iman and Madonna, has doubled in the past ten years. As a result, we late thirties/early fortysomething parents are realising that we could be in our seventies or eighties when we become grand- parents, which is a big leap when the current average age for becoming a grand- parent is 54.

We are a generation that has benefited from having unusually young parents. Historically, the Sixties was an anomaly. An economic boom combined with traditional morality meant that couples married and had kids very young. Women born in the 1940s had their first child at an average age of 23, which is precisely what my mother (born in 1940) did. That meant that even though I was 38 when I had my son, she was a fit and energetic 61 when she became a grandmother.

So to be a young granny, should we middle-class older parents be encouraging our children to start having families earlier than we did? Well, possibly, but there is some good news. The truth is that the “new old” are not like the “old old”. Just as we are redefining middle age and middle youth, experts say that we can indeed expect to live longer and be healthier longer into old age than before.

Professor Tom Kirkwood, of the Institute for Ageing and Health, in Newcastle upon Tyne, says: “The average life expectancy for women today is about 85 and continuing to rise by about two years every decade. And of course the number of centenarians is also rising. In addition, older people tend to rate their health as excellent and seem to be staying younger longer. I think we all know of 70-year-olds who seem as youthful and vigorous as the 50-year-olds we remember from our childhoods.”

My fear, however, and that of many people I know, is that when our time comes we will be too old to be involved grandparents. There is plenty of evidence that children benefit emotionally from close contact with their grandparents. Especially during family break-ups or during the tricky teenage years. But other people my age with older parents find themselves pretty much on their own.

“My mother is 80 and has dementia,” says Caroline, 44, whose children are 6, 4 and 2. “I am no more exhausted by my kids than any other mother. But the added pressure of not having help from my mother, and having to sort out her care, to visit her and to carry that burden is shattering. And I feel so awful for my children that they don’t have a grandmother. My husband’s parents are also in their seventies and though they try, my father-in-law has had two heart attacks and finds having young children around extremely tiring.”

Ruth, 41, and a mother of two children aged 3 and 1, says: “I feel incredibly envious of people who have young, lively parents who have the children for weekends or just help out. My mother had me quite late — she was 38 — so she’s nearly 80 now. She’s healthy but old and frail, basically.”

The inability of grandparents to cope physically with their grandchildren can have a big impact on families emotionally and financially. Recent figures from the Daycare Trust put the value of childcare by grandparents at £1 billion a year. More than a third act as childminders and half of all women who work fulltime rely on their parents to look after their children. This saves families about £26 million a week in childcare fees.

And the chances are that as healthy older parents we will become healthy older grand- parents. For many of us having children and wanting to stay around for them is a powerful incentive to give up smoking, to eat better and to exercise, so further increasing our chances of living longer. “I want to keep healthy so that I have the energy I will need as an older mother having a younger baby. It’s really important that when I’m 50, and my daughter is 10, that I can still run around and do things with her,” says the TV presenter Trinny Woodall, who was 40 when she had her daughter Lyla.

Astonishingly Patricia Rashbrook, very publicly pregnant with her fourth child via IVF with donor eggs at the age of 62 (having had her third at 44), has a 90-year-old father to be grandfather to her baby. So perhaps, even though we will be older grandparents, it won’t mean that we can’t be active and involved if we choose to be.

Unless of course we decide to spend our old age driving round America, going to parties, furthering our careers or even, though it won’t be a path I’ll be taking myself, having more babies.

Getting better with age

A study at the University of Manchester and Leiden University Medical Centre showed that women who gave birth to

one or two children in their thirties and forties had a greater chance of living into their eighties.

A study of centenarians in Europe, by the Max Planck Institute of Demography in Germany, found the same relationship between longevity and fecundity. Why? Apparently having a child in middle age doesn’t make women live longer. Rather, Perls says: “The factors that allow certain older women to bear children — a slow rate of ageing and lower susceptibility to disease — also improve a woman’ s chances of living a long time.” In other words, late fertility is a marker for good health and youthfulness. And it is hereditary.