By Lisa Mitchell
BBC News Online
It used to be people made a huge deal of celebrating the Big 4-0. Then they would slink into old age, pausing only to collect a round of applause and a carriage clock at retirement. Now the corks are popping a decade later and the phrase 'Life begins at...' has new meaning for many.
Bargain Hunt's Tim Wonnacott has more energy and fun now he's 50
The start of a downward spiral into old age or time to kick-start your life? Whichever way it is approached, turning 50 can be momentous.
The sinking feeling you will never be seen as young again, the undeniable wrinkles and the looking back on life and wondering "What have I achieved?"
"It is a time of reflection," says clinical psychologist Myra Hunter, who - like Prime Minister Tony Blair - has herself just turned 50.
"People review their lives. It's natural to take stock and look forward."
Dr Hunter says people turning 50 now are having the same reactions as people a generation ago did at 40.
These post-war 'baby boomers' have more expectations at 50 than their parents.
"Go back 50 years and life stages were more predictable," says the consultant at St Thomas's Hospital, London.
"People of 50 would have parents in their 70s starting to get ill and their children would have left home.
My wife said it was a midlife crisis but I felt I needed a new challenge
"Now people marry later, get divorced, change jobs - their experience is more varied.
"People are more optimistic now about what they're capable of."
Hitting 50 gave Bargain Hunt presenter Tim Wonnacott a new lease of life.
Offered the chance to stand in for David Dickinson, Wonnacott stepped down as a chairman of auctioneers Sotheby's after 25 years and took up television presenting full-time.
"My wife said it was a midlife crisis but I felt I needed a new challenge," he said.
"It's been incredibly good for me - I feel a good deal younger and have more energy.
"Remembering all those contestants' names and what they've bought gets the grey matter going - I'm much sharper than before.
"And it's great fun. For years at Sotheby's, I saw things sell for thousands of pounds. On Bargain Hunt, amusing things are being bought and sold for just a few pounds and I'm finding it fascinating."
Fear at 40
Wonnacott admits he did go through some trauma about growing older, but that was at 40.
"The lines had started and I could feel myself physically deteriorating. At the same time my children were still pretty young and that's a lot of responsibility.
"Now I'm 50, they're delightful to have around.
"I feel sorry for Tony Blair having a baby at this age. He says it keeps him young but I doubt he's feeling it."
The prime minister admitted to Saga magazine - whose readership is aimed at the over-50s - that he was "dreading" his fiftieth birthday.
Turning 50 is like swimming in England in August - hitting the water is awful but once you're in it's lovely
Editor Emma Soames said his reaction rang a lot of bells for her readers.
"Turning 50 is like swimming in England in August - hitting the water is awful but once you're in it's lovely.
"It's a great time of life when you're in your stride and are more confident.
"There's a saying, "If youth but knew and age but could". Your 50s are the only time in your life when you do have the knowledge and you still have the drive and ability."
It is not surprising Mr Blair is reluctant to embrace being 50.
50 'not funny'
In a depressing message on the Department for Social Security's website the PM addresses over 50s as "older people" and then lumps them in with pensioners.
It appears he is not alone in his dismal assessment that 50 is nothing to shout about.
Clintons Cards say they sell 37% more 40th birthday cards than 50th and more depressingly, while 40% of 40th cards are funny, only 20% of 50th ones are.
Whichever birthday you are facing, youthfulness is a "state of mind", says business psychologist Marie Mosely.
"Midlife crises happen from 35 to 50 and the most important thing to ask yourself is what do I value most and what do I want to do before I die?
"And then do something about it."