If you're in your 30s
All of a sudden, you've gone from celebrating Father's Day with your dad to celebrating it with your own kids.
"As you go from being an everyday athlete to being more of a weekend warrior, you need to be smarter about your workouts," advised Whitlock.
Now is the time to start laying down the foundation for healthy aging. You may still feel like a kid, but your joints know better. Work hard and play hard, but spend some extra time in the gym working on joint stability.
Do your chest presses with dumbbells while lying on a stability ball instead of using a barbell and bench, for example. This will help keep you injury-free on your weekend adventures.
Challenge: Train for a sprint triathlon.
If you're in your 40s
The demands at work have ratcheted up, and those kids who once gave you hand-drawn Father's Day cards now expect you to pay for four years of college. As a result, your stress levels are through the roof.
Physically, you need to do something to counteract the effects of sitting at your desk hunched over a keyboard for 10 hours a day. Tight hamstrings, hips and shoulders will lead to knee, back and neck problems if they're not addressed. And mentally, you have to figure out a way to deal with your stress levels.
A consistent yoga practice will open up your body, and studies have shown that mindfulness training can help reduce stress-based issues.
Challenge: Find time for yoga at least twice a week.
If you're in your 50s
If you've noticed some changes in, well, things, you're not alone.
"As you age, your testosterone levels start to decrease," said Whitlock. "Fortunately, one of the ways to naturally boost testosterone levels is by increasing muscle mass."
Studies show that strength training will not only help maintain testosterone levels, but will also help boost the levels of other sex hormones. In the gym, this means lifting like you were back in high school again.
As long as your form is good and your joints can take it, go heavier than you may have been lifting in the past and concentrate on the larger muscle groups -- your chest, back, glutes, quads and hamstrings.
Challenge: Lift heavy.
If you're in your 60s
You may not be as stable or as balanced as you used to be. Balance depends on three things: your vision, the canals in your inner ear and your body's own internal software. As vision and the workings of the inner ear deteriorate over time, you need to do as much as possible to work on your overall stability.
"Working on balance and body awareness will not only help keep you injury free in your day-to-day life," said Whitlock, "but if you're a golfer or a tennis player, you'll also see improvement on the course or the court."
Challenge: Start doing tai chi.
If you're in your 70s, 80s or 90s
Just because you don't have enough fingers to count the number of kids, grandkids and great-grandkids you have doesn't mean you have to act like an old man -- or walk like one.
"People talk about 'losing the spring in your step' when you age, but that's actually what happens," cautioned Whitlock. As the calves get weaker, walking becomes more of a shuffle.
In addition to making you appear older, it's also a major cause of trips and falls, according to studies. To work your calves and improve your balance, stand on your toes for 10 to 15 seconds three or four times a day.
Challenge: Walk a mile every day.