Raymond Francis, an author and health-maintenance expert, is 77. But a Doppler ultrasound, which measures the elasticity and cleanliness of arteries, indicates that his arteries have a biological age of 25. Francis said his goal is to have the arteries of a teenager by the time he is 80.

Growing old often is associated with heart disease, memory loss and arthritis. Though there is no elixir for eternal youth, avoiding certain unhealthy habits can help you feel younger throughout your later years.

SUN EXPOSURE

Unprotected exposure to the sun is one of the biggest culprits for premature aging of the skin. Beyond causing wrinkles, unprotected exposure to ultraviolet light can lead to melanoma and other types of skin cancer. Protect your face and body from unnecessary physical aging by applying sunscreen throughout the day.

UNHEALTHY DIET

Even if you have the most beautiful skin in the world, you could be old on the inside, according to author and journalist Lauren Kessler. Her book, "Counterclockwise" (Rodale, 2013), deals primarily with aging from the inside out.

Kessler and Francis both stress the importance of avoiding processed foods. Try instead to stock your kitchen with fresh foods that are high in omega-3 fatty acids and vitamin B-12. Spending hundreds of dollars for unusual "superfoods" or supplements is unnecessary when you can get the same nutrients from an orange or a piece of salmon, Kessler said.

INACTIVITY

Exercise can benefit your body weight and energy levels. It also improves digestion, immune system function, muscles and brain activity, Kessler said.

Going to the gym for an hour each day is a good start, but Kessler advocates integrating exercise into your life. For example, the next time you meet up with friends or co-workers, try going for a hike instead of meeting for dinner. The same amount of socialization can be achieved in healthier ways.

TOXINS

The key to lasting health is knowing how to keep just one cell healthy, Francis said. To do this, you must keep the cell free from toxins that can interfere with its function.

Toxins can be found in anything from laundry detergent to toothpaste. Specifically, try to avoid heavy metals such as lead and mercury, as well as bisphenol A (BPA) and flame retardant, Francis said.

Knowing what to look for and what brands to avoid can greatly reduce your toxin intake. In "Never Be Sick Again" (HCI, 2002), Francis's first book, he devotes an entire chapter to toxins. Francis believes he is living proof of his method's success. He said he has had just two colds in the past 27 years.

SLEEP DEPRIVATION

Your body is a self-repairing system. As you sleep, it will repair the damage it has endured throughout the day, Francis said. Adults should sleep for seven to nine hours each night, according to the National Sleep Foundation.

SMOKING

Smoking causes premature aging by increasing a person's risk for ailments such as heart disease and lung cancer. Secondhand smoke also can be detrimental to health.

The rate at which you age is within your control. Making subtle, healthy lifestyle changes can have profound effects.

"The latest thought is that 30 percent of how we age is genetic and 70 percent is habits and lifestyle," Kessler said. "That's huge."