"One of my patients, a tv actress, came into my office the other day pretty upset," recalls Ava Shamban, M.D., a dermatologist in Santa Monica, California. "She is in her early thirties and has noticed that people have suddenly started casting her as a killer. Her face has changed—the corners of her mouth have started turning down just slightly, and it has given her a much harder look."

It's one of life's crueler jokes. Your thirties are supposed to be your best years: You finally have a handle on your relationships, you're on track at work, you've built a respectable shoe collection. In other words, you've gotten your act together. But it's at this exact moment that life throws you for a beauty loop. "Thirty is often the age when you start to see your looks changing...and not for the better," Shamban says.

Those first deep wrinkles begin to emerge, turning once-charming crinkles into full-on crow's-feet. High school-style acne may decide to make a comeback, or crop up for the very first time. Damage from decades old summer-camp sunburns can begin to emerge as brown spots and saggy skin. A formerly plush ponytail may become a scrawny shadow of its former self. You get the not-so-pretty picture.

Hormones are one of the main culprits for all these maladies. "The majority of hormone changes start around age 30," explains Beverly Hills endocrinologist Eva Cwynar, M.D. Because we're engineered to have babies in our teens and twenties, our reproductive hormone levels begin to taper off in our thirties, which can result in hair loss and funky skin conditions. Human growth hormone, or HGH, begins to peter out too, which means cells don't turn over as quickly. This puts the brakes on collagen production, giving way to dull, slack skin. "When we're 18, our growth hormone levels may be around 800. By 30, they've dropped to 150. It's normal. It's just how we age," assures Cwynar.

And if you spent your twenties on a beach chair with a glass of chardonnay in one hand and a cigarette in the other, those vices will begin to show, quite literally, on your face. "You start to see the cumulative effects of hard living on your skin in your thirties," says dermatologist Rebecca Giles, M.D., owner of FIX skin clinic in Malibu, California. Everyone is going to see some changes in her thirties due to hormonal shifts, "but for those who have been tanning, drinking, smoking, and eating poorly, the problems are going to be worse."

To keep you looking gorgeous throughout your thirties and way beyond, we asked dermatologists to identify the major skin and hair issues you're bound to encounter during this decade and offer ways to keep problems at bay. If you've already detected some beauty erosion, don't fret. There's still time to undo the damage.