What wrinkles? As Reese Witherspoon, Keri Russell and Michelle Monaghan turn the big 4-0, beauty experts tell FEMAIL how to keep your skin glowing into your 40s and beyond

Let's be real: Most celebrities have great skin - it's part of the job description. But three stars with impeccable complexions celebrated their 40th birthdays this week, causing us to take a closer look at their flawless faces.

Upon further inspection, it's clear that Reese Witherspoon, who turned 40 on Tuesday, and Michelle Monaghan and Keri Russell, who celebrated their birthdays on Wednesday, have barely aged since their 20s.

So besides good genes, what's their secret? FEMAIL went to straight to the pros - including Michelle's esthetician - for answers.

Time flies! Actresses Reese Witherspoon (left), Michelle Monaghan (middle) and Keri Russell (right) turned 40 this week. Apparently no one told their skin that, though, because it hasn't changed since their 20s

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Walk the skin care line! 40-year-old Reese's skin looks as good today as it did 20 years ago. Swipe left to see how the Oscar-winner looked in 1996

SKIN CARE IN YOUR 20s

Biba de Sousa, a master esthetician in L.A. who specializes in anti-aging, knows good skin.

In addition to Michelle, her celebrity client list includes Miley Cyrus and Emily Blunt. 

Before speaking about taking care of your skin in your 40s, however, Biba stressed the importance of starting with good habits in your 20s - so those of you who feel like you have a way to go before the big 4-0, take note.

They're common-sense lessons, but worth repeating: Living a healthy life, using sunscreen, not doing drugs, eating well, exercising, thinking positively, and limiting alcohol are essential to good skin, she said.

Before you get all 'been there, heard that', listen to her specific skin care tips for the younger side of the Millennial generation.

'Young skin does not necessarily need much exfoliation, but it does need vitamins like C, A and B,' said Biba.

'There is a type of B vitamin called niacinamide that has healing properties. It’s a new thing in skin care, I would say in the past five to seven years, and you can see it now in upscale and advanced skin care products.'

Shop products with niacinamide:

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Mission not impossible! Thanks to regular facials and consistent sunscreen use, Michelle has kept her complexion as glowing at 40 as it was at 26. Swipe left to see what The Path actress looked like in 2002

SKIN CARE IN YOUR 30s 

Your 30s are when you should see a professional and adopt a serious skin care program, Biba said.

You know how major exfoliation wasn't necessary in your 20s (see above)? Well, it's a game-changer in your 30s because cellular turnover slows from 28 to 30 days to a full 45 days.

'This is where the serious exfoliation – chemical peeling, microdermabrasion, microneedling, dermaplaning and enzymatic exfoliation - starts,' said Biba. Without it 'you’ll notice the skin gets dull, there’s a buildup on it and it’s lifeless, so we want the skin to shed and reveal a dewy complexion'.

Biba recommends that her clients at this age get an exfoliating treatment every two to three months, in addition to using a less-intense, at-home exfoliator that works for their skin type.

Michelle Monaghan once told Byrdie.com that she gets a facial every six weeks and uses the Dr. Lancer Polish Exfoliator once a week to 'slough off all the dead skin'.

Shop at-home exfoliators: 

Skin type-specific corrective serums and acids can also help keep skin glowing during this time. 

'Everybody is given specific skin characteristics, and everybody has a different childhood,' Biba said. 

Problems that may need to be addressed could be any one - or a combination of - the following: fine lines, pigmentation, redness, sensitivity, acne, extreme oiliness, thick skin and acne.

Keri Russell, for one, has used Baszicare Artius Eye Vital Supreme, which at $320 is pricey, but it targets fine lines, under-eye circles and puffiness. 

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Is Keri a skin care spy? The Americans actress, 40, looks exactly the same in 2016 as she did in 1998. Swipe left for proof

SKIN CARE IN YOUR 40s 

One of the biggest skin care mistakes New York City-based dermatologist Julie Russak, M.D., sees women in their 40s make is continuing to use the same regimen they used in their 20s and 30s. 

Biba agrees. 'Women often look very similar at 35 as they do at 45, she said. 'So they are not taking the physiological changes seriously because they still look the same.'

Dr. Russak broke down those physiological changes: 'Perimenopause sets in your 40s, which means "around menopause" and refers to the time period during which a woman's body makes its natural transition toward permanent infertility (menopause),' she explained.

'The collagen and elastin fibers break, thicken, stiffen, clump together and lose their elasticity. This results in wrinkles and aging lines. 

'Skin shows more drastic changes as cell turnover slows down further and collagen, elastin and moisture level in the skin drop. Actual wrinkles and deeper expression lines develop as a result.'

These internal changes mean 'you may need to be more vigilant in your skin care', said Dr. Russak. 'Continue to use sunscreen, daily moisturizing and more advanced skin care products. Look for skin products with vitamin C and peptides to help the connective tissue under the skin to produce collagen. Eye cream is now a definite must.' 

Biba echoed that advice and added that 'facial treatments have to be intensified, usually every month.' 

Shop skin care products with vitamin C:

As for products to stop using, Dr. Russak advised staying away from products with benzoyl peroxide, which can over dry the skin and disrupt the skin’s barrier and harsh, soapy cleansers that pull moisture from the skin.

Both skin pros recommended the use of supplements at this age. 'Now it’s not a joke, it’s not even like a "I’m on a health kick,"' said Biba. 'This is survival.'

According to Dr. Russak - who often directs her patients to NeoCell’s Collagen Beauty Builder - as hormones level drop, collagen production slows drastically. 'Collagen protection, stimulation and supplementation is key at this age,' she stressed.  

Essential fatty acids like Omega 3s are another must. 'They help the body produce oil and distribute it evenly on the skin,' Biba said. 'So if you don’t have them, the skin will be angry; it will be dry and dehydrated.'

The epidermis loses lipids (fatty substances) that keep skin moisturized and plump, Dr. Russak explained. 'Skin becomes thinner as we age and loses a lot of the underlying fat layer that keeps young skin smooth and supple.'

Shop hydrating products:

THE BOTTOM LINE 

Biba, who is in her 50s, said women should welcome the transition into middle age. 'It's a very exciting age for women because we get to know all kinds of secrets of life,' she said. One of those secrets though, is that it's 'us who hold the key to our happiness' and that happiness comes from doing the right thing. 

In terms of skin care, doing the right thing isn't that hard. 'Nobody’s asking you to wake up at 4 in the morning and do a crazy routine,' Biba said. 'It’s more about not doing the wrong things. That’s it.'

'Aging is like death and taxes,' she continued. 'It will happen - and it’s not bad. People are so scared of it, but aging is wonderful.'

Witherspoon agrees. Earlier this year she told Harper's Bazaar, 'It's a great thing getting older. You are who you are; you say what you mean. And if your face falls down a bit? I kind of enjoy that!'