Fitness in your 20s
A change in recovery between workouts is one of the first signs of progressing age . In our teens and 20s, the brief 24 hour window that previously allowed the muscles and energy to recharge fully, no longer applies. In the later 20s you are not elderly nor out of shape, you simply have to acknowledge that your body needs more recovery time. And if Roger Federer can declare, “At age 27 your body starts talking to you”, then this is also true for the rest of us.
This a consideration regardless of how tough, strong and active you are. If we do not listen to our bodies, we risk over-training, even in our 20s. Lightening up the workout on day 2 or simply switching yet another run for a swim, maybe all we need to prevent over-training
Fitness in your 30s
When we hit this milestone it might be wise to skip exercise every 3rd day. A monthly massage, quality sleep and a diet rich in Omega 3 fatty acids, including cayenne pepper, cumin, ginger, and Holy basil which would help decrease post-workout inflammation. Our metabolism slows in the 30s and our caloric requirements drop, so it’s important to tailor one’s eating habits to reduced needs. Avoid starchy refined foods – those dreaded empty calories that have few nutrients but pack on the weight.
According to AncientMinerals.com, Magnesium intake is important to maintain energy levels.  Also, pumpkin seeds, brazil nuts, and almonds are good sources. 
Fitness in your 40s
Kids and work commitments we are more likely to impact the regularity of our workouts. Getting regular exercise may be more of a challenge than worrying about recovery. Inter alia responsibilities and sleep changes may affect our hormone status. In terms of nutrition Cherry juice and vitamin D are good pre-workout foods. Best Health Magazine describes how cherries are a powerful anti ageing and anti-carcinogenic food,  while Vitamin D is the perfect bone health nutrient. Focus should also be on anti-oxidants. According to EatrightOntario.ca you can get antioxidants from cherries, green tea and vegetables , and vitamin C from citrus fruit, spinach, peppers,  and E rich foods. i.e. vegetable oil, nuts, and green vegetables.  For Vitamin D, you should have sunlight exposure and fatty fish, milk, and egg yolks. 
Fitness in your 50s
It is imperative that folks in their 50s stay active. It helps if they have been active all their lives, but often they are playing “catch up”, trying to regain their health and shape with what is now a slower metabolism sometimes accompanied by health conditions. Everything from digestion, muscle, bone and mental health benefit from regular exercise. Exercise must be appropriate and should account for thinner joint surfaces and changes in the physiological makeup of muscle. Recovery, possibly 2 full days, may be needed to allow these tissues to regenerate. Low impact exercise such as swimming, cycling are options and if one is able, jogging. Resistance training will ensure muscle mass. This is vital since loss of muscle is the most significant area to be compromised as we age. Resistance work will also provide positive stress on bone thereby encouraging the body to keep laying down bone tissue.
Detailed in the U.S. National Institute of Health, bone loss can be minimized by ensuring Vitamin D intake but your focus should also be on Calcium  (sourced from dairy products, sardines, and kale) and vitamin B12 (liver, beef, dairy).  Some older adults do not produce enough hydrochloric acid to absorb nutrients from whole foods, in which case a multivitamin is a wise supplement. While it is imperative that folks in their 50s are active and can sustain fairly intense workouts, they must bear in mind that they are more susceptible to injury.