Though this formula is not always accurate it provides a great way to know your working zone.
I am going to answer this as I answer anything for people under the age of 18, it really falls on the parents shoulders.
Teens are old enough to make decisions but many times the decisions are best directed by the direction of a parent, family member or mentor. Let me know explain:
If a teen has no direction they are left to make decisions on their own so it is completely up to their decision on whether they choose to be active or sedetary. Go outside or play video games, it is up to them. However....
As a parent the teenager is still shaped by the decisions and example the parents leave. If the parents are engaged in their kids lives, active outside then the chances their teen will remain active are also high.
Does the parent make time for their teenager? Does the parents do things as a family that involves activity outside or if they do things as a family does it involve food and sitting?
My entire point is most teenagers are thinner than what they will be as they age so for most teens weight is not a huge consideration. However, if their example is laziness and poor eating habits then when they get older they have no idea how to fix a weight problem that may occur. If they are a teenager and their parents are active and eat healthy then the teenager has a much better chance of remaining active and healthy as well.
We are still our teenagers example!
Physical activity is important for adolescents for many reasons. Physical activity decreases their risk of developing childhood obesity, which in turn dramatically reduces their risks of developing diabetes and heart disease later in life. Regular physical activity in children has been shown to improve test scores and academic achievement, promote stronger bones and muscles, improved motor skill development and improves social skills. Physically active children have also been shown to have higher test scores and are more likely to graduate high school as well as attend college.
Weight training is important for everyone over the age of 12 with physician consent. Interval training is a great way to incorporate weights and cardio in one workout. There are many different ways this workout can be designed and it can be done up to three or four times a week with great results in as little as three weeks. Some have even claimed to notice a difference after just one work out. Younger adults have a unique advantage to getting these habits started earlier and extremely reducing risk of a whole list of health problems people that don't exercise will experience as early as 30 years of age.
Make it fun. Exercise is any activity that stresses the musculoskeletal system or the cardiorespiratory system. Exercise is not limited to weight training or running on a treadmill. Make exercise a game that your kids will enjoy. Ride bicycles, play catch or soccer in the local park. You can also, take your kid swimming as this is an excellent form of exercise. Getting your kid involved in after school or community sport leagues is another way to exercise.