1. Sunscreen Why it’s a big deal: Sun exposure is the number one cause of premature aging, Diaz says. Ultraviolet light speeds the formation of lines, wrinkles, and sun spots, a double whammy since sun-induced damage hampers skin’s ability to repair itself, too, he adds. “SPF is all we have, other then clothing, to protect our skin from the sun’s aging effects—and skin cancer,” says Ruth Tedaldi, MD, a Boston-based dermatologist. The 40+ upgrade: Be choosy about your ‘screen: “As we age, our skin can become more sensitive,” says Debra Jaliman, MD, assistant clinical professor of dermatology at Mount Sinai School of Medicine. Look for a physical SPF formulation—one with active ingredients such as zinc oxide and titanium dioxide, like Derma e Antioxidant Natural Sunscreen SPF 30 with clear zinc oxide ($20, dermae.com). These sunscreen formulas have fewer chemicals and tend to be less irritating, she explains. Wear it: Every morning after cleansing and applying serums or lotion. It should be the last skin care product you put on before applying makeup.   MORE: 8 Plants That Repel Mosquitoes Naturally
  2. Retinol  Why it’s a big deal: Tedaldi (and pretty much every derm we’ve ever spoken to) agrees—there isn’t another all-around age-reverser as powerful as retinol. But here’s what happens when you don’t wear it: Your collagen production slows and new cell growth decreases, resulting in thinner, sagging, wrinkling skin that can’t reflect light like it did when it was taut and smooth, so it’ll look duller, too, Diaz says. Retinol stimulates collagen production and cell turnover to keep chugging year after year, so these aging issues stay in check.  The 40+ upgrade: If you’re a retinol newbie, apply an over-the-counter option like RoC Retinol Correxion Sensitive Night Cream ($22, walgreens.com), every other night. It’ll be gentler than prescription-strength retinol but powerful enough to make a difference in your skin, says Joshua Zeichner, MD, director of cosmetic and clinical research in dermatology at Mount Sinai Hospital in New York City. If you’ve already been using retinol for a while, or you’ve noticed a plateau in your skin’s improvement, it’s time to visit your derm for a prescription version, Zeichner says. “It all depends on your skin, but it could take anywhere from a few months to a few years after you start using it before you need to switch,” he adds. (Enjoy glowing skin with the groundbreaking hormone-balancing plan in The Hormone Reset Diet.)Wear it: At night, just after you wash your face and before you slap on any night creams.  MORE: 8 Things That Happen When You Finally Stop Drinking Diet Soda
  3. Antioxidant-rich serums Why it’s a big deal: Free radicals, which come from UV light and pollution, damage skin cells’ DNA, so they’ll look older, sooner. Antioxidants help block the damage from wreaking havoc on your cells so your skin stays bright and youthful, Jaliman explains. The 40+ upgrade: Be smart about when you apply it. An antioxidant-rich serum is best smoothed on in the morning, since you’re most vulnerable to free radicals when the sun is out. Tedaldi recommends green tea extract in particular for women past their 30s—research in the Journal of Cosmetic Dermatology suggests that the antioxidant has wrinkle-reducing talents, too. We like John Masters Organics Green Tea & Rose Hydrating Face Serum ($26, johnmasters.com), which stars a certified-organic version of the ingredient.Wear it: Every morning just after cleansing your face. 
  4. Moisturizer Why it’s a big deal: The oil glands in your skin decrease as you age, so your skin retains less and less moisture and, instead of being smooth and plump, is more likely to sag and form fine lines and wrinkles, says Jaliman. But all is not lost: “A moisturizer provides the hydration needed to prevent these changes,” Diaz says. The 40+ upgrade: In your 20s, your skin had more natural moisture, so you might have been able to skip moisturizer here and there, Jaliman says. Now, you need to keep it on ‘round the clock, and make sure what you’re putting on has hyaluronic acid, glycerin, peptides, or ceramides, all of which help your skin retain moisture and temporarily plump it, so skin instantly appears more youthful, Diaz says. Peptides, in particular, provide the skin with the nutrients it needs to rebuild and repair itself, he adds. Try Derma e Deep Wrinkle Peptide Moisturizer ($39, dermae.com), which packs both peptides and glycerin. Wear it: Day and night, over any serums or medications (because it’s thicker and will keep them from sinking in if you swap places).