Six years ago, after a lifetime of competitive running that started even before his days as a college track athlete, Dean Smith’s knees finally announced that they couldn’t take the pounding any longer.

So the Lone Tree resident switched to rowing, both in waterborne sculls and on indoor machines. Since then he has won scores of medals in national and international rowing contests.

At 86, he apparently feels he has no time to lose.

“I was never a star athlete until I got into my 40s,” Smith says. “I started winning, and that really motivated me to work even harder.”

Smith is among a cadre of senior citizens who are proving that fitness can be cultivated and held, even among folks who have carried AARP cards in their wallets for decades.

“The main thing is, don’t stop,” Smith says. “I don’t care how old you get or fragile you become, you have to keep the juices flowing.”

Along with the cardiovascular benefits, a mix of aerobic and resistance training can lower blood pressure, improve circulation and increase bone density, an especially important issue in the aging.

“Exercise never stops benefiting you,” says Mary Schiavone, head trainer at the downtown Denver YMCA. “We have seniors here who have lowered their blood pressure and even been able to go off some of their medications because of exercise.

“It’s something you can do at any age.”

Although maybe not at Smith’s level.

The Illinois native has two rowing machines in an upstairs room in his house and often rows with a trainer. He is also a member of the Rocky Mountain Rowing Club, which has a boathouse for its sculls at Cherry Creek Reservoir.

Smith’s workouts are intense. Most of his daily sessions last about 90 minutes, and he can typically row the equivalent of 10,000 meters in that time. A laptop computer linked to his rowing machine gives him feedback on his performance, along with a visual display of a simulated boat so he can mark his progress on a make-believe waterway.

“I like to train hard,” he says. “I pretty much use maximum effort the whole time.”

It has paid off.

From 2006 to 2009, Smith won medals at the World Rowing Masters Regatta. He believes that his age actually gives him an edge. “As you get older you’re more focused and can work out more efficiently,” he says.

Whether you have never participated in an exercise program or are returning to one after a layoff, Smith recommends taking it easy at first.

“Start out gradually,” he says. “Don’t try to recoup everything in the first couple of weeks. If you gradually work at it and build on it, your skills will come back.”

At 60, Denver attorney Cliff Eley is too young to be considered a senior citizen, but he’s close enough to see the horizon.

Eley began exercising in earnest in 2011, after years of sporadic gym-going interrupted by long layoffs.

“I noted that the older I got, the more I felt the effects of not exercising,” he says. “Walking down the steps, my ankles and knees hurt. I realized it wouldn’t get any better and that I had to get back into exercising.”

Eley hired a personal trainer, a move he recommends if you can afford it. He does 30 minutes of cardio work six days a week, working to get his heart rate up to 150 beats a minute at some point in the routine. He lifts weights four times a week.

His trainer, he says, pushes him to points he doubts he would reach by himself. All his efforts have made routine activities easier.

“When my wife wants me to do a chore in the yard, it’s no longer a problem,” Eley says with a laugh. “I don’t mind weekends like I used to.”

If you can’t afford a trainer, consider working out with others, whether a friend or a group. A partner will help keep you motivated and allay boredom, one of the enemies of a sustained workout routine.

You might join a Silver Sneakers program, a senior-oriented exercise program offered at many YMCAs, gyms and recreation centers. (The one at downtown Denver’s YMCA at 25 E. 16th Ave. meets four times a week.)

Above all, it’s rarely too late to start working out.

“There’s no reason to think, ‘OK, I’ve reached this age and I’m overweight and out of shape, so that’s the way it’s going to be,'” Eley says. “If you’re morbidly obese, it’ll take a while before things get better, but they will get better.”

William Porter: 303-954-1877, wporter@denverpost.com or twitter.com/williamporterdp

Tips for starting an exercise regimen

1. Check with your doctor and get an assessment of whether you’re ready for exertion.

2. Start out slowly, especially during the first month.

3. Learn some basic stretching exercises and perform them before and after you work out.

4. If you have never exercised, consider scheduling a session or two with a trainer.

5. Working out with a partner makes exercise for fun.

6. Try and get a good night’s sleep. That’s a key part of recovering between workouts.