The idea that if you act the part, you can become the part holds true—sometimes. My husband and I acted like we had money in graduate school, and it certainly didn’t make us rich. But in the case of youth, acting young makes you feel younger.You only need to observe children, from infancy on up, to get some ideas of what it means to act young, and you will find that your body responds in kind when you act the part. In watching my own kids, here are a few things I’ve learned about movement and staying young.RESEARCH: Think Young, Look YoungMove every chance you get.  When babies are awake, even fetuses in utero, they move.  They may not stun you with their grace and coordination, but their movements develop critical systems: neurological, musculoskeletal and cardiorespiratory. And as these systems develop and kids reach a few months and beyond, you can see one goal in their eyes—to get somewhere and do something. And if you try to stop them, you invoke the look and the scream that announces you have interfered with a self-evident right. They should be able to climb onto the table, hang onto the drapes, turn off the TV over and over, run out the door, worm their way out of their car seats and run with wild abandon through the mall or the aisles at church.So when do we go from thinking it’s a thrill to turn the TV on and off again and again to thinking it’s one of life’s great inconveniences when the remote is missing and we have to get up to change the channel? Somewhere along the line, movement stops seeming like something we must do and starts becoming an inconvenience. Rather than an opportunity for freedom, it’s a chore.Yet movement is as critical to maintaining our body systems as it is to developing children’s. When you move, it develops and maintains your brain and neurological system, even triggering the generation of new brain cells. Movement develops and maintains your muscles, connective tissues and bones. No wonder older parents claim that their kids keep them young. Chasing after them and playing with them truly does keep them young.The lesson here? Move whenever you get the chance. If you have a choice of moving more or less, move more—walk farther instead of taking the shortcut, take a bike instead of the bus. You get the idea.COLUMN: Six Habits for Busy ExercisesChallenge yourself. Recognizing the risk of incriminating myself, I’ll admit that I have kids who run the wrong direction on moving walkways in airports and on escalators. As my sons and daughter came stumbling off the escalator a few months ago, I whisper-yelled, “Why do you do that when I told you not to?!” My sons replied in unison, “’Cause it’s fun!”  My daughter, pointing to the other escalator, breathlessly replied, “I like to see if I can beat the people who are walking up that one.” Young people naturally take joy in physical challenges. Our six-year-old spends hours counting how many times he can jump the rope without a mistake, and then he works on crossing his arms and hopping on one foot in unison.Kids’ bodies and minds develop because of the challenges they give themselves. Your body and mind responds that way, too. When you challenge yourself by moving faster and longer, your cardiorespiratory fitness increases. Your heart and blood vessels deliver more oxygen to working muscles, and your body uses that oxygen more efficiently to produce energy. You have more youthful energy, and your insides and outsides actually look younger, too: your arteries, your brain, your skin. Working to develop new movement skills, such as jumping rope or dancing, and cultivates poise and coordination, along with the other brain- and heart-health benefits.My one-year-old constantly seeks physical challenges in the strength and coordination arena. That’s how kids overcome gravity to sit, crawl and stand. They challenge themselves to the point of collapsing, stumbling and falling over and over. Kids push themselves to the brink. When I lift weights, our one-year-old joins in. But whereas I use a dumbbell that is about 20 percent of my body weight, he takes one that is a third of his and hoists it over his head with both arms. Luckily, he has me to grab the weight from his hands as he careens backwards. The challenge he pursues makes him stronger and more coordinated. If you challenge yourself the same way, by moving against significant resistance, you also become stronger, and your musculoskeletal system resembles a much younger person. Your muscles are bigger and stronger, your body is leaner and your bones maintain their density.MORE: Boost Your Brain With ExerciseThe lesson here? Do activities that are not easy for you. Go a little faster than what is comfortable. Force your muscles to overcome significant resistance. Try some activities that test your coordination and that require your practice, such as dance, martial arts, aerobics, or a new sport. Simply challenge yourself.Play like a kid. Children’s natural play involves movement. At lunch yesterday, our four-year-old daughter was pretending she was a dog. She wanted me to hold pieces of food high above her head, and she had to jump to get it. Wouldn’t we all be in much better shape if we played this game? At a recent track meet for our 13-year-old son, I sent the five other kids to a nearby field to play. Within two minutes, they were playing freeze tag and having a much better time than the competitors or the onlookers. I decided to join in and had a blast. About 20 minutes later, others who had been watching us decided to participate. It was an exhilarating, youthful experience.The lesson here? Play. Play with your kids, your dog and your spouse.To stay young, look to the young and pattern your movement habits after theirs. Embrace every opportunity for movement, seek out and participate in activities that challenge you physically and engage in physical play. You’ll not only feel younger in spirit, but your body will also feel, look and be younger.QUIZ: How Active Are You?