As your body changes when you age, so should your exercise routine.

There are practical considerations everyone needs to make when it comes to exercising. What physical condition are you in and what are you capable of when you start exercising? How do you avoid injury? What is motivating you to get in shape?

The most important aspect of fitness is motivation. You can know all there is to know about exercising and fitness, but if you are not motivated to get in shape, to make that change, then none of that information makes any difference.

When starting out, get a good understanding of where you are in your life. Make sure your decisions, in terms of physical activity, are in line with your day-to-day routine. For example, a college-aged guy may have time and motivation to play intramural football or spend hours in the gym. Then you have a guy in his 40s with a busy job and a family. He’s going to have to make adaptations to his schedule in order to fit in his fitness routine.

When starting out, pay close attention to how your body feels after workouts. Is it invigorating or does it create days of exhaustion and pain?

One of the things that happens when we get older is we recover slower. This isn’t just from injury, this is your body’s adaptation to physical activity of all kinds. The older you are, the more important the slow and steady approach to exercise becomes to avoid injury and intense discomfort.

The other reality is, while you can gain strength and speed no matter your age, you are never going to have the capabilities of your younger self. Even if you train as hard at the age of 40 as you did at 20, you’re still not going to be as fast and strong as you once were.

For example, if you adopt a running regimen, something I think you can do at any age, starting out, it’s going to be easier for young men. They can typically run long, hard and fast and the next day not be in as much discomfort as a guy in his 40s or 50s. That’s not to say if you’re 50 you can’t become a marathoner. You absolutely can. You just need to take your time getting in shape.

That’s one of the reasons why in qualifying for the Boston Marathon (something I did after the age of 40) the time requirements are more forgiving the older you are. I had to train myself properly to compete; it wasn’t something I jumped into and decided to do in a few months. When adopting a running regimen, a person can start off just walking. Then move to running a block, walking a block and so on until you become a regular runner.

Regardless of your age, I think everybody should be a weight lifter. It’s best to get instruction to learn how to lift properly. If you are using proper form and advancing at a rational pace, you will get stronger and more muscular, and that’s going to protect your body against injury well into old age.

Being unable to protect your body in your later years is often referred to as compressed morbidity.

These types of exercises help you live longer and aid in delaying infirmity. By lifting weights in line with your present ability and patiently working to get stronger, a lot of other physical activities are going to become open to you.

When you start lifting weights, free weights have advantages that weight machines don’t, but that doesn’t mean machines are bad. They are especially good for beginners and the older population who are concerned about their abilities to lift weights. If you are capable of using free weights, again, I advise getting proper instruction.

Using nothing but machines is a lot better than doing nothing at all. It’s sort of like the difference between jogging and running. Jogging is going out for a slow run, so that would be lifting with machines, where running is going out and pushing it.

You will get more results from focusing on free weights, but it requires more intensity and there is a greater risk of injury.

There are not a lot of activities you can’t do in your 20s and even into your mid-30s based on your relative health. Gymnastics is the only activity that is probably too late to begin. Other than that, the entire wide world of sports is open to you. But that doesn’t mean you should go out and do something extreme. If you are just starting to get active in your 30s, you need to be cautious and see your doctor before starting an exercise program.

Physical decline generally starts in your 40s, but if you’ve been in shape most of your life, it’s still not that pronounced. You need to use more caution and may need to eliminate specific types of activity, or at least do them in a more cautious fashion – like skiing or martial arts. At 46, with 21 years of weightlifting, I know that my biggest lifts are behind me. But, if you’ve never lifted in your life and you started at 46, then your heaviest lifts are probably in front of you.

Move your focus to delaying aging. You’re starting to think more about having an active, healthy, pain- and injury-reduced retirement — one where you don’t spend your retirement on the couch. You want to be in a place where you are active, healthy and fun. That’s not to say fitness goals are not important. I don’t want to discourage someone in their 50s from being ambitious, but those ambitions need to be tempered with wisdom and caution.

The older you are the more the pool is your friend. Exercises in the pool greatly reduce the impact on your joints. But the pool is more of an upper-body workout than lower body, so if you are of an advanced age, don’t just swim. Consider adding brisk walking to everything you do.

James Fell is an internationally syndicated fitness columnist and author. His works have appeared
 in the Chicago Tribune, LA Times, TIME Magazine and AskMen. Fell is also a National Strength and Conditioning Association certified strength and conditioning specialist.