The big 5-0. A half century. Two score and 10. Whichever way you look at it, 50 is a symbolic birthday and a major staging post in life. But reaching the age of 50 - with the worries over health, wealth and happiness, menopause and mid-life crises that sometimes accompany it - is a peculiarly modern problem. One hundred years ago, you would have been lucky to get there. Men born in 1900 had a life expectancy of 48 years and women were likely to live to be 52. For most of us, 50 is unlikely to be the halfway point in our lives (scientists doubt that life expectancy will reach 100 for several centuries, if it ever does), but it can feel like it. As life expectancy in the West nudges 80, it is more like the midpoint in our adult lives.
Quinquagenarians (that's what they're called) are past the irresponsibility of youth but before the inevitable onset of old age. More than at any time in history, 50-year-olds are caught in the generation gap between their parents and their offspring, able to see what they have left behind and what's to come. And to cap it all, they can find themselves responsible for the care and financial upkeep of both sets of relatives.
It's a dilemma felt by Don Watson, an education manager who turned 50 a couple of years ago and who typifies this stuck-in-the-middle situation. He was married at 25, then the average age to tie the knot, so his half-century also coincided with his silver wedding. By this time, his two children were facing A-level exams and degree finals respectively. His parents lived hundreds of miles away, and he found that phone calls home increasingly concerned care arrangements and home helps.
As his 50th birthday approached, Don found himself becoming preoccupied. "I thought about it fairly obsessively in the two to three months leading up to it. As I walked to the station each morning, there was a particular point in the journey at which I used to think, 'I've got 20 more days to go'. But once I was over that waterfall of 50, I never really thought about it - I was being swept downstream and I didn't need to worry about it.
"It's a significant birthday - it's halfway to 100. You are supposedly in your prime of life and in some respects you are. But you can't fool yourself any more that you are young-ish. You are definitely middle-aged at 50."
His wife, Wendy, head of a successful primary school, is "quite confident" about her own imminent half-century, and top of her wish list is a sports car. "That's what happens when you are 50 - it's a time to indulge yourself. But you also tend to do a lot of reflection about where you've got to and where you are going. You know that this is as good as it gets. For people who have not been very successful in life, it can probably be quite depressing.
"It's a difficult time of life. You are quite well off compared to when you were young, but you are paying for kids to go through university. It's not just about material wealth, it's about family life - if you've got kids, you can measure yourself to a certain extent by how well they are doing."
In some respects, there's never been a better time to be 50 in Britain. Over-50s make up a third of the population but have 80 per cent of the wealth - they are the generation courted by everyone from insurance companies to cruise operators. If turning 50 can be a personal milestone for an individual, it could become something of a millstone for society. By the middle of this century, people aged 50 and over will form the majority of the population.
A recent report by the Economic and Social Research Council noted that while "in the past the lifestyle of those in their 50s was characterised by permanence and stability", today's 50-year-olds are a lot less easy to categorise. "Improved health conditions will underwrite their assumptions that they will live for at least another 30 years. More of them will see themselves as 'young' and be more inclined to engage in a diversity of active, creative leisure pursuits. More self-focused, hedonistic attitudes will emerge as they imitate the lifestyles of the young," says the report.
It also predicts an increase in the divorce rate and overall poverty, as retirements last longer and the pensions system comes under pressure. But the prospects are not all bad. After the ups and downs of youth, the plateau of middle age can be a steadying influence, according to Paul Watkinson, a 52-year-old English teacher. "You move into the grandparent generation - I have a grandson just starting school - which gives you a kind of accumulated wisdom and optimism.
"Turning 50 is an emotional thing. It might sound maudlin, but when you enter late middle age, you start to address death. When you are younger you feel immortal, like you can go on forever. But now people you have known all your life are dying. Every day, I open the obituary page and see someone I know of has died."
For him, the coming of middle age also marked a new lease of life. After a working life spent in creative industries, he became a teacher three years ago, at the age of 49. "Kids treat me differently than they would a younger teacher. And I don't think I'm as jaded as someone who has been teaching for 28 or 30 years. Training to be a teacher was nerve-racking. Walking into a lecture hall with 18-year-olds and not knowing if I could meet the challenge was fresh and exciting. I'm still learning in my job and it's great."
All names have been changed. See Sean Coughlan's money column, page 30.I Don't Feel Fifty - website for the young at heart: www.idf50.co.uk Lifestyle magazine for 'the new generation': www.over50s.com health, travel, money advice and offers: www.saga.co.uk