When we’re young, exercise can enable us to run a race after an all-nighter or snowboard on a diet of Doritos. But as we age, fitness has a much more far-reaching impact, boosting our energy levels, preventing injuries and keeping us mentally sharp.

Aging causes muscles to lose mass, bone density to thin and joints to stiffen — affecting our balance, coordination and strength. At the same time, hormonal shifts and persistent low-level inflammation can set the stage for chronic diseases such as cancer, cardiovascular disease and diabetes.

And the changes start earlier than you might think. Muscles begin to shrink in our 30s and continue their downward spiral in midlife, with up to 25% of their peak mass gone by the time we’re 60.

But there’s hope: Exercise can stall muscle loss, cognitive decline and fatigue.

“It’s never too late to start exercising, and it’s never too early,” said Chhanda Dutta, a gerontologist at the National Institute on Aging.

You can’t just start dead-lifting 150 pounds at the gym, though. Start slow, experiment and gradually amp up the intensity.

Experts suggest trying exercises that target one or more of four categories of fitness, all of which deteriorate with age: flexibility, balance, endurance and strength. Preserving function across these domains can stave off injury and disability, keeping you active and independent longer.

Lower body: squats and stairs

During exercise, “injuries happen when you’re fatigued, and your muscles can’t react as quickly,” said Dr. Brian Feeley, chief of sports medicine at the University of California, San Francisco. Squats help prevent this fatigue by strengthening the large muscles in your lower body while moving multiple joints at once, which improves overall endurance as well as balance and coordination.

Feeley suggests doing three sets of 10 to 15 squats four times a week. To further challenge your balance, do them with one foot or both feet on a pillow. Or to focus on strength, squat while holding free weights — close to your chest to start or extended in front of you to work your core more.

If you loathe squats, try climbing stairs, which is adaptable to different fitness levels, said Dr. Maria Fiatarone Singh, a geriatrician at the University of Sydney. Start by walking up and down the stairs, and graduate to sprinting or wearing ankle weights.

Heart rate: Nordic walk

As a cross-country skiing enthusiast, Dr. Michael Schaefer, a rehabilitation physician at University Hospitals in Cleveland, loves Nordic walking — an exercise using ergonomic poles that uses the same movements. No snow required.

“Nordic walking is unparalleled as an aerobic exercise because you’re not just using the major muscle groups of your legs and hips, but your core, shoulders and arms, too,” Schaefer said.

The regimen lowers blood pressure and improves the body’s use of oxygen. And when you traverse hills or uneven ground, you’re strengthening your ankles and challenging your vestibular system — a sensory system housed in the inner ear that enhances balance and coordination.

“Start with 15 to 20 minutes three times a week and work up to one hour,” Schaefer advised.

The basic movement — walking, using poles to propel your movement — can take some getting used to, but online videos or your local Nordic walking group can get you started. The key is to swing your arms as if they’re clock pendulums, keeping the elbows relatively straight and planting your pole behind you and pushing off as your opposing leg strides forward.

Upper body: Try hanging

If Katy Bowman, a kinesiologist, had her way, everyone’s New Year’s resolution would include a trip across the monkey bars.

“It’s such a primal movement and uses all these parts of our upper body” that otherwise don’t get used very often, said Bowman, author of “Rethink Your Position.”

Hanging from a horizontal bar enhances grip strength and shoulder mobility, strengthens the core and stretches the upper body — from the chest to the spine to the forearms.

As with any exercise, it’s best to progress slowly — start by hanging on a bar with your feet supported on a box or chair so that muscles unused to carrying a load can become accustomed to bearing some tension. From there, proceed to an active hang, in which your shoulder blades are retracted and pulled down (as if you’re about to start a pull-up), your core and arms are engaged, and your hands are about shoulder-width apart.

Add a slight swing front to back or right to left to work the core and spine even more. Or mix up your grip — hands facing away from or toward you, or one of each — to emphasize different muscles. An underhand grip, for example, loads the biceps more than an overhand grip, which works the lats.

And you don’t need fancy equipment to hang. Bowman suggested creating a hanging station in your home with a “$20 doorway chin-up bar that doesn’t take up much of a footprint.” Since she’s installed one, she said, she’s noticed a “radical” increase in her upper body and grip strength — which is linked to a decrease in all-cause and cardiovascular mortality. A little goes a long way, too: Begin with 20-second hangs, twice a day, working up to a full minute.

“Frequent, shorter hangs distributed throughout the day are your best bet for making progress,” Bowman said.

Core and hips: Use a slider

If you work at an office or a desk, all of that sitting can do a number on your hip flexors, the muscles that help you bend your knees toward your waist and stabilize your spine. And hunching over a desk shortens the muscles in the chest while lengthening those in the back, contributing to muscle strain and weakness of the lower neck, shoulders and upper back.

To counteract this, Nicole Sciacca, a mobility specialist in Los Angeles, pairs mountain climbers with sliders — small disks on which you rest your hands or feet that slide freely on the floor (or use paper plates). Training on an unstable surface increases the intensity of an exercise, forcing you to engage your core — especially the diaphragm, transverse abdominis and pelvic floor — to maintain position.

“It’s great because it asks everything along the front side of the body that’s been sleeping at a desk or in a car to get stronger,” Sciacca said.

If you’re new to working your upper body and core, Sciacca suggests holding a simple plank for 30 seconds. Once that’s comfortable, position your feet on the sliders, assume the same position and work to keep yourself stable.

To progress, move one foot in under your body until your knee reaches your chest. Slide that foot back out while your other foot comes in. Continue alternating your feet for up to three rounds of eight reps, keeping the core strong and the back straight.

Improve flexibility: Foam roll

Tala Khalaf, a physical therapist at Stanford University in Palo Alto, Calif., thinks of fascia — a system of connective tissue that wraps around our muscles and organs — as the Cinderella of orthopedic medicine. For years this tissue, which is studded with sensory nerves and can look like a sheath around the outside of muscles or found within them, toiled away in obscurity, ignored and minimized.

But research in the past decade has lifted up fascial tissue as a crucial component of the musculoskeletal system. As we age, fascia becomes less pliable and elastic, which contributes to back pain, stiffness and a limited range of motion.

Khalaf, who is also a faculty member at Stanford’s Orthopedic Physical Therapy Clinical Residency Program, said one solution is foam rolling, which massages out the fascial kinks and improves flexibility. Best of all, the basic moves are simple and time-efficient. Typical areas to roll include the calves, thigh and back. Experiment to see which exercises provide the most relief.