For many, these are also the years for partying, eating what you want and still looking and feeling fine - but it should be about building foundations for future health and fitness, says Professor Greg Whyte OBE, a world-leading sports scientist who coached celebrities such as David Walliams and Davina McCall in their Comic Relief challenges.
"In your 20s you are at the peak of your powers, whether you're female or male. This age needs to be about building strength and muscle - because this will biologically start to decline when you hit 35 - as well as building cardiovascular fitness."
Building muscle now helps develop a good metabolic rate, protecting against weight gain, and will also protect your bones, joints and posture as you age, says Matt Roberts, celebrity personal trainer and author of new book Younger, Fitter, Stronger: The Revolutionary 8-week Fitness Plan for Men.
"A fair amount of research shows collagen is more readily produced when you have more activated muscle mass, so it's anti-ageing."
Professor Whyte recommends 30 minutes of moderate intensity physical activity on most days of the week, including strength endurance classes, such as circuits, and cardiovascular activities such as swimming, running, cycling or rowing.
Crucially, this is the decade to find an exercise you love. "Do you love being indoors or out, with a team or alone, in a class or not? You want to build up the healthy addiction to exercise," he says.
30s
Try: squash, tennis, pelvic floor exercises
Busy, time-poor and likely to be spending more time in a sedentary job, this is a time to integrate energetic, multi-tasking sports into your routine, says Roberts.
"Tennis or squash played at high intensity is good for getting a muscular, cardiovascular, flexibility and co-ordination workout along with a stress release within one 45-60 minute commitment weekly."
He also advises "exercise snacking" - running home or doing a short, 25 minute run in your lunch hour.
This is the decade that children are most likely to come along, so women should be paying attention to pelvic floors with planks, side planks, pilates and yoga.
"Try the bridge - lying on your back with your feet flat on the floor, knees bent and doing pelvic lifts up and down," Roberts advises. 'This is using your glutes but you're actually clenching the entire pelvic floor at the same time."
Muscle naturally starts to decline at this stage, while circulation reduces and we start to lose flexibility and elasticity in the soft tissues - so your weekly rugby or 5-a-side game may start to produce more injuries. Leave longer recovery times after exercising, and incorporate recovery workouts such as yoga into your weekly routine.
40s
Try: Marathons, weights, pilates
For the sandwich generation, hectic lifestyles can get in the way of exercise, and stress itself can trigger the start of the dreaded middle-aged spread.
"The stress hormone cortisol forces the body to access its sugar resources so it doesn't burn away as much fat," Roberts explains.
"This can accumulate as visceral fat around your belly and organs, which can then increase your risk of type 2 diabetes and high cholesterol."
To bust stress try brisk walking, spin classes, jogging and swimming.
Meanwhile, the trend for endurance activities such as long-distance cycling, Iron Mans and marathons among middle-aged people is actually quite welcome in biological terms, says Dr Polly McGuigan, senior lecturer in biomechanics at the University of Bath.
"The first muscle fibres we start to lose as we get older are our fast twitch muscle fibres, which are the ones we use when sprinting, or heavy lifting. But our slow twitch muscle fibres are recruited more during endurance activities."
However, take it slow, use a proper training programme and keep working on muscle strength to avoid injury, she says.
Women in their 40s may be approaching the perimenopause and menopause, and fluctuations in oestrogen may already be affecting bone density, says Prof Whyte, so try racket games such as squash, tennis and badminton, or "good old-fashioned load-bearing jogging and fast-paced walking".
As for the ever-expanding spare tyre, Roberts recommends lifting heavy weights three times a week to really elevate metabolism: think weighted squats, lunges, bench presses and dead lifts.
"Combine that with high intensity interval training [HIIT] three times a week with long rest intervals."
Core development work is important, now too, he adds: try pilates, or anything that works the internal abdominals.
50s
Try: Boxing, golf, circuit training
"A lot of people think they need to slow down in their 50s, but we need the opposite," says Professor Whyte.
"Of course you need to be careful and build up gradually, but if you haven't started yet, this is your last chance to ensure someone isn't wiping your backside, aged 70 - I usually find that's a great motivator."
Take a leaf out of Carol Vorderman's book - last week the 58-year-old revealed she does 400 squats a week.
To keep up muscle mass and heart health, try cardio activity such as walking and swimming three times a week, fitting in functional training, too, such as squats, deadlifts, bench and chest presses and push-ups. A weekly session of yoga or pilates is an excellent way to protect against falls in older age.
Look for exercises that increase mobility and co-ordination, to work the brain and also your balance, Matt Roberts advises.
"Anything that involves throwing, catching or hitting, such as golf or tennis as well as boxing with pads, is great for making the brain work and for coordination."
In the gym, try moves that involve drills with rhythm and changing directions such as circuits.
60s
Try: Power walking, gardening, swimming
Health conditions such as high blood pressure, heart disease or a previous heart attack are more common at this time of life, and it's natural to worry about exercising, says Roberts.
"Take your doctor's advice. What we do know is doing nothing makes you get worse, and exercise can help increase the strength of the heart after an attack."
You may be offered an exercise ECG or stress test, involving walking on a treadmill or using an exercise bike while your heart rate is monitored, to allow your doctor to set some boundaries.
"This time of life is really all about keeping mobile and on your feet,' says Roberts.
"Keeping your cardiovascular system conditioned at this time is crucial, alongside building stability in the hips and lower back and mobility in the shoulders and knees, to help keep problems at bay."
Moderate intensity cardiovascular work such as jogging, power walking, gardening, dancing or swimming a few times a week is important, he advises.
"But you also need to do some interval training to condition the heart and lungs. So that could be walking fast up a relatively steep hill for 45 seconds and then strolling back down again, and repeating that six to eight times."
70s and beyond
Try: Ballroom dancing, table tennis, yoga
Retirement from work means this is a great time to start social exercise such as ballroom dancing, says Roberts. "Anything involving numbers is great for coordination, and the social factor helps keep the mind young too.
"People with Parkinson's, for example, do very well with social exercise such as dancing or table tennis."
Back pain can be common at this age, and mobility and strength work is important to ensure flexibility and maintain posture. Hatha yoga delivers stretching work in the hips, quads, hamstrings, and back, but with a low risk of injury.
Consider simple stretches and balances at home, too. Helen Mirren, 73, and the Duke of Edinburgh, 98, are said to be fans of a 12-minute exercise plan devised for the Royal Canadian Air Force, which includes arm circles and hopping on one leg.
In a study, Dr McGuigan prescribed a similar programme to a group of 65 to 80 year-olds, incorporating five exercises to be done for one minute each, twice a day. The activities could all be done at home with no need for equipment - such as standing up from a chair, standing knee bends, or marching on the spot.
After four weeks, Dr McGuigan's team saw a one to two per cent increase in quad size (the gold standard measurement of muscle growth) and an overall five to six per cent increase in strength.
"These people had never exercised before, which really does give new meaning to the words, 'it's never too late,'" says Dr McGuigan.
