Now that we have that unpleasant fact out of the way, let's get to some good news: You don't need to be a genetically blessed model or have a plastic surgeon on speed dial to keep your body in remarkable shape. Instead, just dive into the closest thing we have to a fountain of youth: fitness.

With the right workout strategy, one targeted for your age and physiology, you can fight back against the inevitable slowdown, build muscle and keep your weight in a happy place. Whether you're blowing out the candles on your 30th-birthday cake or planning a big 5-0 bash, follow our decade-by-decade game plan, based on advice from scientists, nutritionists and fitness pros, to get the body you want right now.

In your 30s

What's happening: You are still close to peak potential for being in incredible shape (see soccer goalie and "Dancing with the Stars" alum Hope Solo and tennis star Serena Williams). But your body is in the early stages of rebellion.

"Starting in their 30s and early 40s, women lose about 5 pounds of muscle in every decade," says Wayne Westcott, instructor of exercise science at Quincy College in Massachusetts.

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That's a body bummer, since the more muscle tone you have, the leaner you look (because muscle is more compact than fat) and the more calories you burn at rest (because it takes more energy to maintain muscle than fat). In fact, according to data from the National Health and Nutrition Examination Survey , a normal-weight woman is likely to gain 5% of her body weight per decade between the ages of 25 and 45. Even so, says Westcott, if you're active, "you're still in control."

Fitness game plan: It's essential to stay at your ideal weight during this decade to set yourself up for success later, when biology makes it more challenging. That means cardio is key.

A 2010 Harvard study found that premenopausal women in their 30s who rode a bike for more than four hours a week, for instance, were 26% less likely to gain more than 5% of their starting body weight over the course of 16 years.

You can also get good results from 30 to 40 minutes of running, a dance class or whatever other heart-pumping exercise you like four or five times a week, says Wendy Kohrt, professor of medicine at the University of Colorado's division of geriatric medicine.

Mix in muscle-building exercises to jack up your metabolism even further and tone up: Do as many reps as you can of a strength move (say, push-ups or lunges) for one minute. Follow that up with two minutes on a bike at a challenging pace. Continue alternating, varying the strength move each time, for 15 minutes.

Test yourself! To check your cardiovascular fitness, stand facing a 12-inch-high bench or box. Step up on and off the box for three minutes.

Then count your pulse for a minute. It should be between 58 (excellent) and 110 (average). If you retake the test in your 40s, your pulse should be between 60 and 112 (in your 50s, between 63 and 118).

Nutrition game plan: To lose weight at this age, a moderately active 5-foot-5, 150-pound woman should consume around 1,500 calories a day (but no fewer!), says Jackie Newgent, author of the book "1,000 Low-Calorie Recipes." That can lead to a loss of 5 pounds in about 5 weeks, 10 pounds in about 10 weeks and 15 pounds in about 15 weeks.

The secret to regulating your hunger hormones and making chip binges less likely, according to Cynthia Sass, Health contributing nutrition editor, is eating on a regular schedule. That means having breakfast within an hour of waking up and spacing remaining meals three to five hours apart.

Also, make sure you're getting 1,000 milligrams of calcium a day: The mineral is needed to help muscles contract and maintain strong bones. "By the middle of this decade, what you have in your 'bone bank' is what you have for life, but you can build and maintain it," Sass says. Good calcium sources include yogurt, canned salmon (which typically packs more than fresh), almonds and leafy greens such as kale.

In your 40s

What's happening: Thanks in part to hormonal fluctuations, your metabolism starts to sputter as you head into menopause, causing weight gain. If you skip out on strength training, you'll likely lose another 5% of your muscle mass, Westcott says.

But (deep breath!) these are just "ifs" and general rules. In fact, the average age of female participants in the Ironman World Championship is 41.

Fitness game plan: While you need to continue with your cardio, strength training, any exercise that puts progressively greater force on your muscle and bone, is more important than ever for keeping muscle mass and bone density at their peak and fending off unwanted pounds. Think about gradually working up to heavier dumbbells or adding more reps of push-ups. "The key to a better metabolism is breakdown and repair of muscles," Westcott says.

Strength training builds muscle mass and bone density.

Test yourself! To check your overall body strength, get in the plank position. You should be able to hold it for at least 60 seconds. If you're in your 30s, raise the benchmark to 70 seconds. If you're in your 50s, lower it to 50 seconds.

Retest yourself every four weeks after starting a strength program to see how your results improve.

Nutrition game plan: Don't cut your calories any further. But do aim for balance in your meals, advises Sass, by focusing on foods rich in good carbohydrates (like fruit and whole grains), lean protein (fish, yogurt and beans) and healthy fat (avocado, nuts and olive oil). The carbs will give you the fuel you need to stay energized all day, while the protein and fat will help you heal and maintain the muscle that's so crucial to maximizing your metabolism in your 40s, Sass adds.

In your 50s

What's happening: As you go through menopause, your estrogen production declines, accelerating bone loss. This is why you can expect up to a 30% decline in bone mass in your 50s if you skip exercise, setting you up for osteoporosis.

Lack of estrogen has also been linked to weight gain, especially around the abdomen. Then again, there are ways to maintain your fit body. Just ask marathoner Joan Benoit Samuelson, 56.

Fitness game plan: Turn to a combination of regular walking, strength training and stretching to keep yourself lean, strong and safe, since in the next couple of decades, balance issues can lead to injuries. "Stretching not only has an effect on flexibility but also builds muscle strength," Westcott says.

Yoga is a great addition to your routine if you're not already into it: According to a study at the Fred Hutchinson Cancer Research Center, subjects between the ages of 45 and 55 who did yoga at least once a week for four or more years gained about 3 fewer pounds than folks who didn't hit the mat.

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Test yourself! Stand with feet together, hands on hips. Place bottom of right foot on inner left calf, just below knee. Hold for as long as possible. Repeat on opposite side, then calculate your average hold time.

If you can maintain the position for more than 50 seconds on average, you have excellent balance (woo-hoo!). If you crumble after 25 seconds or less, improve your time by incorporating yoga or tai chi into your weekly workout routine.

Nutrition game plan: You should keep aiming for that 1,500-a-day calorie count, and make sure you're eating enough protein. By the time you reach 50, you don't process it as well as you did in previous decades, Westcott says, so you have to exceed the recommended daily amount of 46 grams.

Here's more motivation to get it now: A study at the University of Texas Medical Branch at Galveston found that eating 4 ounces of protein-rich food per meal (as opposed to eating 12 ounces all in one sitting) maximizes your body's muscle-building rate by up to 50 percent. Westcott recommends following up a sweat session with foods like chicken breast or chocolate milk.

Turns out, a glass of red wine could help your anti-aging efforts, too. A recent study in the journal Menopause suggests that having one or two glasses a day may help slow bone breakdown. Cheers to that!