Last week, the Canadian Society of Exercise Physiology (CSEP) released the New Physical Activity Recommendations CSEP_PAC-FactSheet-:

The Guidelines for exercise is comparable to weight control. While the first step for weight control is preventing weight gain before trying to lose weight, first step for exercise guidelines is stop sitting all day long, move around at any occasion like walking, taking the stairs rather than the elevator. Be active for an equivalent and total of .10 000 steps. 
Then, second step, try to include through the day 30 min of moderate intensity exercise for additional health benefits. If you are particularly sedentary, the 30 min of moderate intensity becomes vital. 
Ideally, third step, double that moderate intensity exercise to 60 min for more benefits. 
Ultimately, 4th step, 50% of your more formal 30-60 min aerobic exercise should be high intensity. This is recommanded for optimal fitness. Interval training or circuit training is an efficient method to achieve this part. High intensity training should be supervised by a kinesiologist or other exercise professional.
And this is without talking about weight training as a complement.

Children and Youth (aged 5-17)
How much and how often?
• Children and youth should accumulate at least 60 minutes, and up to several hours, of moderate physical
activity (such as brisk walking, skating or bike riding) each day.
• For very inactive or sedentary children and youth, some health benefits can be achieved through 30
minutes of moderate physical activity per day.
• Some physical activity is better than none, and more is better.
What kind?
• The physical activity should be mostly aerobic, and should include vigorous activity (such as running and
playing soccer) at least 3 days a week, and muscle and bone strengthening activities (such as skipping,
jumping or playing in the park) at least 3 days a week.
• Daily physical activity should be achieved through play, games, sport, work, transportation, recreation,
physical education or planned exercise, with family, at school and in the community.
Existing guidelines – Children (aged 6-9) and Youth (aged 10-14)
• Increase time currently spent on physical activity starting with 30 minutes more moderate to vigorous
activity per day, increasing to 90 minutes more per day.
• Reduce non-active time spent on TV, video, computers games and surfing the internet, starting with 30
minutes less per day, increasing to 90 minutes less per day.

For more information
Weighty Matters: How many minutes a day should you exercise?