It isn't age, it's atrophy. Your body becomes a product of what you don't do with it. If you don't run, after a while you can't run without gasping and hurting. If you don't reach up, after a while you have to get a stepstool, because you can no longer reach up without pain in your shoulders.
But the biggest mistake you can make is trying to undo years of atrophy in just a few weeks. If you want to take up snowboarding, or get back into skiing after a long time off, you're going to need several months of conditioning first. A re-conditioning workout will make your body younger.
Orthapedic surgeon Jeff Harrison once said that he warns his patients who want to be more athletic to start by literally going through the motions.
"Do squats and bench presses without a weight to start. Get your joints used to doing exercises before you add resistance," he advises.
Joint training is especially important if you haven't been active for several months or more. Most joints produce a lubricating substance called synovial fluid. It covers the cartilage at the ends of each bone to prevent friction and wearing of the cartilage. Those who are young or active have a ready supply of synovial fluid; it flows as soon as they start moving. As we get older and less active, the joints take longer to begin producing the stuff. An inactive baby boomer may need a ten minute treadmill warmup before the knees begin the oozing of synovial fluid into the joint.
This is not just true for baby boomers. The high powered executive who spends the day sitting on the way to and from work, then sitting at work and sitting at home after work; the stay-at-home parent who spends the day minding the kids with some minor time in the kitchen---their joints become middle-aged; the ligaments are no longer flexible, the lubricating fluid is no longer produced quickly.
Once joints have adapted to inertia, they are prone to injury from any vigorous activity. So start your reconditioning routine with simple movement exercises, like pedaling a stationary bike. Yes, it's boring, but not as boring as a nine-month recovery from a blown out knee.
For the first two weeks, your workout should start with a stationary bike. While you are pedaling, hold out your arms in front and to the sides, and move your hands in small circles to work your shoulders. Do this for ten minutes at an easy pace.
Next, stand with feet shoulder width apart, hands on hips, and move your upper body around in a circle, keeping your feet stationary. Do one minute in one direction, another minute in the other direction. Repeat for a total of four minutes. This gets your hips joints and spine accustomed to moving and flexing again. The first week, repeat the bike and torso circle routine twice for each workout. The second week, do three reps.
After two weeks, or a minimum of ten workouts, you are ready to graduate to resistance work. Begin each session with a bike and torso rep as a warmup; and remember two important things: when you first begin your return to action, you will need longer warmups, and when you start resistance work, start off light.