ANSWER:
Adults: these people can do 75 minutes of high-intensity aerobic exercise and two or more days of muscle-strengthening exercise. All these are needed to maintain a good physical health, and maintain a strong body.
Children and adolescents: these people don't necessarily have a time limitation, because during this ages, they have a lot of energy, and they are able to perform 60 minutes or more of a combination of aerobic and muscle-strengthening exercises.
Individuals with brittle bones: these people must have individualized exercise routines according to their abilities, beacuse they have brittle bones, and depending of the severity of it they are able to perform certain type of exercises.
Pregnancy: women in pregnancy periods should do 150 minutes of moderate-intensity aerobic exercise, because their pregnant condition don't allow them to perform intense workouts. Also, they need a lot of cardio to maintain a specific weigh and not affect the baby