People exercise for different reasons therefore how much exercise needed varies greatly.

Before you decide on how much exercise you need, you should have an idea on your goals. Are you exercising for physical fitness, weight control or as a way of keeping your stress levels low. This information is very important because it will determine the way you will conduct your exercise. For health issues walking is the best exercise. 
Other reason like lowering your blood pressure, cardiovascular fitness is the option. Aerobics and muscle strengthening activities are what everyone needs. Aerobic activity like walking, swimming, and cycling improves cardio-respiratory fitness. Muscles strengthening activities are also important since they involve the workouts of the legs, hips, back, abdomen, chest, shoulders and arms. 

The number of minutes of moderate intensity physical activity each week depends on how vigorous in intensity the exercise is. Exercising for the whole day is not necessarily important, one hour or less is enough for a perfect exercise. To sum up every activity (Aerobic and muscle strengthening) need a decision before starting. The more vigorous the exercise the less time needed. 

Continuous exercise is the key for best results whether for physical appearance or health issues. Exercise should be taken serious whether if some is young or old because it helps both.  Therefore, before you decide to exercise especially for health issues you should consult a physician and use a trainer to guide you. Some exercise would be too intense for you.