The prime minister's wife Cherie Blair is holding a party on Saturday to celebrate her 50th birthday.
Mrs Blair says she is looking forward to her fifties
Mrs Blair, whose birthday is on 23 September, will host a "low-key" bash for family and friends at Chequers.
She told the Daily Telegraph she had enjoyed her 40s and was looking forward to an "equally fantastic" next decade.
She also defended the decision to hold it on the anniversary of the 9/11 terror attacks, saying it was the only time all her guests could get together.
"The trouble was the only weekend we could get all the family together was the 11th of September so I didn't really want to have the party before my birthday but it was either that or no party at all," she told the paper.
On her actual birthday, Mrs Blair is due to present an award for grandparent of the year and hold a reception for her main charities.
Speaking about reaching her half-century, Mrs Blair told the Telegraph: "Ten years ago I had a party when I was 40 and I have had a fantastic decade in my 40s.
"I am looking forward to having an equally fantastic decade in my 50s."
She did admit that the thought of being eligible for a bus pass at 60 - the same time her youngest son Leo will be able to get a student pass at 15 - was "daunting".
Mrs Blair also discussed her speaking engagements in America, her role as Olympic ambassador and her book, as well as giving an insight into her experience as the wife of a prime minister.
She said the duties of prime ministers' spouses included hosting diplomatic events and having "good personal relationships" with everyone.
Another factor was that her husband was never really off duty.
"The prime minister is never totally on holiday and even on holiday we were accompanied by an office and you have to deal with things every day."
Commenting on Mrs Blair's birthday celebrations, a No 10 Downing Street spokeswoman told BBC News Online: "It is a very small, and low key, private family party.
Mrs Blair has co-written a book, The Goldfish Bowl, about spouses at No 10.
She was also recently ranked the 12th most powerful woman in the world by business magazine Forbes.
The magazine described Mrs Blair as getting "her power both from her own work and from that of her husband" - a factor which has led to criticism in the past.
It noted that not only does she live at No 10 Downing Street as "Britain's first lady" but that she also practices under her maiden name Cherie Booth as a Queen's Counsel with London's Matrix Chambers law firm.
Are you looking forward to your 'fantastic 50s'? Do the baby boomers expect a more active and adventurous middle age? Send us your comments and experiences.
This debate has now closed. Thank you for your comments.
Like Ms. Blair, I turned 50 this year. While it was a bit harder than turning 30 or 40, I also do not yet feel "old." In fact, I am in better shape physically, having recently added running to my exercise routine, and am busier and more productive at work than ever before. I too, expect my fifties to be "fantastic," and beyond that - well, someone once said "old age is not for sissies." Like many boomers, I plan to stay very physically and mentally active, and hope to re-define that decade as well.
Sharon Arnoult, Wichita Falls, Texas, USA
I think in your fifties you can relax more and enjoy yourself. You don't have the crisises of confidence which you suffer in your twenties, or the child devoted selflessness of your thirties and forties and you can just enjoy doing what you enjoy doing. I for one certainly feel more self assured and happier than I ever have!
As I lay on the floor of the exercise studio at my gym stretching my muscles with weights and 'fit-balls' and looking around at my 60 + friends I couldn't help but wonder if our mothers and grandmothers had a better idea as they retired with their knitting. But I wouldn't change my lifestyle for anything - I go to the gym, I dance (not tea dance) I do lots of voluntary work, look after grand-child and still have time to travel.
Jean Reynolds, Waterlooville, UK
I'm definitely not looking forward to my 50s! It'll only be another 10 years before I'm 60, and then it will definitely hit me I don't have long left.
Liam, Derbyshire, UK