Most of us have an arsenal of serums and creams to fight wrinkles -- so why aren't we taking the same all-in approach when it comes to tackling another major pitfall of aging: a slowing metabolism?

According to the Center for Disease Control, the average American woman gains almost a pound a year after age 20, meaning that the fear of turning 40 and being stuck with an extra five, 10, or even 20 pounds is perfectly legitimate. And the longer you wait to develop a fitness strategy, the harder it's going to be to get rid of the pooch.

Skip ahead to find the best workout for your age.

While hormones are part of the equation, especially as your body changes during different phases of life, your weight is within your control. "The reason we get weaker and out of shape as we get older isn't so much about biological factors as it is about lifestyle factors," says Dan Trink, Director of Personal Training Operations at Peak Performance in New York City. "If you stay active, you'll have a strong, healthy body your whole life."

Trink, along with all the experts we spoke with, believes that it isn't about adding specific exercises to your routine as you age -- it's more about adjusting your workouts to your changing lifestyle and hormonal profile. "A lot of the training for different age groups is going to be similar," says Maria Pagano, RD, exercise physiologist and Tier 4 Personal Training Manager at Equinox. "But the reasons behind it are different."

Whether you're looking to amp up your routine to get rid of a few pesky pounds or you haven't worked out in years, it's never too late -- or too soon -- to start. "At any age, the risks of being sedentary far outweigh the risks of pulling a muscle," says Trink. "Getting to the gym is much safer than sitting in a chair for the rest of your life."

You heard the man. Now, are you ready to get off your couch and onto a treadmill? See the best workout for your age now.

... and you're still holding onto the Freshman 15.

Your fitness fix: Get in a routine.

Most recent college graduates are more worried about finding a job and living on their own than they are about their metabolism. But the experts we talked to stressed how important it is to start now. "You want to redefine your quality of life," says Pagano. "Exercise and nutrition should become part of the structure of every day." If you don't have the money or desire to join a gym, there are plenty of other ways to get fit. Pagano suggests going back to physical activities you enjoyed in high school and college -- like organized sports.

Because you're young, you can pack a lot of calorie burn into a short period of time and it will keep your metabolism up for the rest of the day. If you want to drop weight, Trink suggests high-intensity metabolic circuits where you move from station to station doing exercises that combine strength and cardio.

... and you want more definition.

Your fitness fix: Work on your posture.

"Women in their 20s generally have a much different goal than older women," explains Trink. "They're going out, and they want to look good in their clothes." He says that correcting your posture will automatically make you look longer and leaner -- and help you work out better. "It is the fastest way to improve body composition," he says. "You want to identify dysfunctional movement patterns -- very few of us move optimally." A personal trainer can evaluate how your body moves or holds itself incorrectly and teach you how to fix your posture before developing a workout that's ideal for you.

Trink also suggests learning the basics of the gym -- like squats, lunges, presses and pull-downs -- when you're in your 20s. He emphasizes that if you have these in your wheelhouse, you'll be more successful both on your own and in class situations.

... and you're thinking about pregnancy.

Your fitness fix: Focus on your core.

"There are so many benefits to getting in great shape even before you get pregnant," says Trink. "It will give you stamina and stability throughout your pregnancy and the childbirth process, and it will make getting back into shape after you give birth so much easier." Core function is especially important while you're carrying extra weight -- it can minimize back strain -- and learning how to activate certain muscles will make it easier to get them back post-pregnancy.

After you've had a baby, Pagano stresses how important it is to get back to your routine as soon as you've been cleared by your doctor. "There's a small window of opportunity when your hormones will actually help you lose a lot of body fat," she says.

... and you're working like crazy.

Your fitness fix: Work out at work.

If you're intensely focused on your career, chances are you're neglecting your exercise routine -- and you're probably getting a "desk butt." "Someone who sits for hours at a time will have tight hips and overstretched glutes," says Trink. Pagano recommends setting an alarm at your desk (try an email reminder) to go off every two hours. "You need to stretch your upper body and open up your hips," she says. Sure, your co-workers may look at you strangely, but incorporating exercise into your workday will make a huge difference in how you look and feel.

Mini exercise bands are easy to stash in your office, and they're ideal for sneaking in mid-afternoon workouts. Pagano suggests lateral and forward walking with the mini band around your knees. Don't work in the kind of office where this kind of fitness break is acceptable? Join a gym nearby and sneak in early a.m. or even lunchtime workouts.