You know that with each passing birthday, your skin starts to look and feel different. (WTF wrinkle, you weren't there last week!) On top of that, the actual shape of your face changes as you get older, too. Luckily, there are steps you can take to stay youthful. Here, find out exactly what happens to your face in your 20s, 30s, and 40s—and what you can do to prevent some of the major signs of aging.

Your Skin: from Acne-Prone in Your Twenties to Super Dry in Your Forties

In your twenties: Great collagen support keeps your skin supple and gives you a healthy glow. It's not all sunshine and roses for women in their twenties, though. Many experience acne, even if it wasn't an issue for them in their teens, with pimples popping up on the lower face and along the jawline in particular, says Doris Day, M.D., a board-certified dermatologist who specializes in laser, cosmetic, and surgical dermatology in New York City. To treat acne, use an anti-inflammatory product with niacinimade, a form of vitamin B3, suggests Heidi Waldorf, M.D., director of laser and cosmetic dermatology at The Mount Sinai Medical Center in New York City. (Get rid of pimples faster with Fat & The Moon Pimple Mud from the Women's Health Boutique.)

How you care for your skin in this decade will determine what kind of damage you deal with in your thirties and forties—which is why it's so important to wear sunscreen year-round. Waldorf also recommends including an antioxidant product in your morning routine, like Caudalie Polyphenol C15 Serum ($62, us.caudalie.com), to stop burgeoning fine lines from turning into wrinkles.

RELATED: How to Undo Years of Sun Damage

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What the lines on your face looks like at 35 (left) and 45 (right)

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In your thirties: "Uneven skin tone with broken blood vessels and sun spots start to become more of an issue, along with lines between eyes and crows feet," says Day. Not getting carded anymore at bars? This is also the decade when lifestyle habits from your earlier years start to show on your face. Hyperpigmentation, such as melasma, which are brown patches that often occur on the forehead, cheeks, and chin, become a big concern for women in their thirties. Melasma is usually a result of sun exposure, but women who are pregnant or are on birth control can experience it, too, due to a change in hormones. Look for an exfoliating cleanser and a night cellular restoration product to help with cell turnover and pigmentation. If you're pregnant or breastfeeding, stay away from retinol and switch to something less irritating, like the Neostrata Skin Active line of products.

In your forties: You may notice your skin starts to get drier, leading to more pronounced lines and wrinkles. Waldorf suggests using a moisturizer like Caudalie Premier Cru the Creme Riche ($158, us.caudalie.com) to help improve dehydration; it contains peptides, antioxidants, and hyaluronic acid to correct dark spots and brighten skin.

RELATED: 5 Ingredients to Avoid if You Have Dry Skin

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What the fat under your face looks like at 35 (left) and 45 (right)

Your Face: from Full and Perky in Your Twenties to Slim and Angular in Your Forties

In your twenties: At this point in your life, the subcutaneous fat (the fat below the surface of your skin) around the apples of your cheeks is firm and perky. Sigh…can't you stay this way forever?

In your thirties: "You also see early signs of volume loss, mostly in the mid-face and chin," says Day, due to less collagen and hyaluronic acid production. That means you may notice that your face is starting to look slimmer and more angular, which can make you look older and more tired. So this is a good time to add some topical collagen and hyaluronic acid to your beauty routine to keep some plumpness. Applying a serum after exfoliating to help lock in moisture, like Ole Henricksen Truth Serum Collagen Booster ($48, olehenricksen.com).

RELATED: 3 New Anti-Aging Treatments You Really Oughta Know About

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What the bones of your face look like at 35 (left) and 45 (right)

In your forties: At this point in your life, your face starts losing even more of that subcutaneous fat you had so much of in your twenties, although you won't lose it equally in all areas. "Although there are variations due to genetics, in general we lose the deep fat pads in the mid-face and those of the temples and the front of the ears first," says Waldorf. Then we lose [fat] around the mouth and chin and along the jawline. It looks like the skin is falling, but actually our faces are deflating." What's more, your skin also loses the ability to bounce back as quickly, causing skin to sag.

When it comes to your nose, now's the time the tip tends to dip downward. "This is partly due to lack of soft tissue support and also a result of the upper lip retracting downward and the nose following," says Waldorf. "The top of the nose at the bridge often sinks, causing a horizontal wrinkle, in addition to the vertical lines between the brows."

If you're looking to help restore some fullness, Waldorf says fillers are the best option. "Patients are often worried about getting over-filled," she says. "However, when done correctly, fillers allow us to create a very natural appearance, more like how the patient looked before. Patients say that they are told they look rested. I call it the Benjamin Button effect. People say, 'Every time I see you, you look younger!' but no one is saying you look done."

All illustrations are courtesy of Galderma, a global dermatology company.