In her late 30s, Cameron Diaz recalls how journalists would often ask her if, as an actress, she was scared about turning 40.

"They were saying, 'Aren't you afraid that the death of your career is imminent because you don't look 25 any more?'"

She writes this observation in her exploration of the ageing process in The Longevity Book, which follows on from her hugely successful nutrition and exercise tome, The Body Book.

The book sees her embrace and celebrate ageing, often citing scientific evidence around what we might expect as we get older, and how we can live better, stronger, healthier lives as we age.

"Why is everybody so scared of ageing? I feel better than I ever have," says Diaz, now 43. "It's not just physical. I feel different mentally as a human being."

Today, Diaz - whose film credits include The Other Woman, The Holiday, Charlie's Angels and My Best Friend's Wedding - fully accepts the fact she doesn't look or feel the same as she did 20 years ago, but is enjoying each era as it comes.

"I'm not trying to have the best body in Hollywood. I'm very happy maintaining my wellbeing, which is a priority in my life. That's important to me.

"As you get old, at any point in your life, the only way you can have success is if you focus on what you gain, not what you have to give up.

"Let's push the mid-life crisis off a bridge and throw ourselves a party instead. The mid-life celebration: a personal holiday that celebrates the journey we've made to get here, and the unexpected places we have yet to discover."

It certainly sounds like Diaz - who celebrated her first wedding anniversary with Good Charlotte guitarist Benji Madden in January - is enjoying her own mid-life celebration.

"My husband is the most loving and meaningful relationship I have ever had. He is one of the pillars of my wellbeing and we get to build our life together," she happily states.

They married at home in Beverly Hills in January 2015, after being engaged for only 17 days, and Diaz is often seen out and about with her sister-in-law, designer and TV personality Nicole Richie, who is married to Madden's brother Joel, Good Charlotte's lead singer.

She's reportedly decided to take some time out of the spotlight to enjoy married life. Today, she remains tight-lipped about the subject of starting a family, preferring to keep the focus on her book.

"I just think it's a privilege to get older. Not everybody gets there. I've lost plenty of friends, who died in their 20s and 30s. We're looking at ageing as a bad thing, but if you're doing it, you are really lucky.

"We're living the longest we've ever lived. I can live longer, but I don't just want to be alive. I want to spend the next half of my life healthy in body, mind and spirit."

So how does she see her 40s and 50s panning out?

"I don't even know if I'm going to be acting. I don't know what's going to happen, but I haven't given up on acting," she admits.

The star, who counts Drew Barrymore and Gwyneth Paltrow among her close friends, has a sunny, optimistic outlook, fostered during her happy childhood in a working-class neighbourhood of Long Beach, just south of Los Angeles, where money was tight but affection was plentiful.

Raised, along with her elder sister Chimene, by their Cuban-American father Emilio (he died of pneumonia in 2008, aged 58) and Anglo-German mother Billie, she attended an inner city state school.

Holidays were usually camping road trips; i t was an outdoor life, where kids would ride their bikes or play in the street.

After working as a model, aged 22, Diaz auditioned for The Mask, which threw her into the spotlight, later becoming one of Hollywood's top female earners.

Unlike the many actresses who bemoan the lack of good roles for older women, Diaz has a different perspective.

She has said: "The most interesting parts are for women who are over 40. We don't see it that way, because they're not the sexy parts. Look at the Oscar nominees in the last decade.

"We're not giving those women enough credit for what they're accomplishing, which is beautiful performances."

In the age-obsessed movie industry, she admits she has tried all sorts of age-defying make-up combinations over the years.

"I've visited my fair share of dermatologists' offices exploring their anti-ageing arsenals, from creams to lasers to Botox and fillers, all for the sake of maintaining a look of youth and beauty."

She continues: "Believe me, I know that it's easy to get caught up with what you see in the mirror and use it as a metric for how well you are ageing.

"But don't be fooled - just because you look younger than your friend, doesn't mean your body isn't experiencing some wear and tear. This ageing thing is a process, and we all have our own individual journey through it.

"It wasn't so long ago that I could skip a few days of working out and, while my energy level and mood might be affected, my body was still pretty resilient. It only took a few days of hard workouts to snap it back into place and regain a feeling of strength.

"It doesn't seem like anything snaps back into place as it used to," she reflects.

She practises what she preaches though: Transcendental Meditation on a daily basis, regular exercise, healthy food and lots of laughter and rest.

"If I don't sleep well, if I miss breakfast or eat something that looked rich and delicious on a menu but turns out to be too rich and delicious, if I miss my workout because I have a stack of meetings that seem more pressing at the moment - well, I suffer for it.

"One of the reasons I wrote this book," Diaz adds, "is because I believe that we would all be a lot happier, feel a lot better, heave a big sigh of relief, if we could just answer, 'How old are you?', with the truth. Without fear. Without hesitation. Without shame."

:: The Longevity Book by Cameron Diaz and Sandra Bark is published by Harper Thorsons, priced £16.99. Available now