The frequency, duration, and type of exercise necessary to achieve optimal health depends upon several factors including the intensity of effort, as well as individual needs, goals, and limitations.  As of 2007, the American College of Sports Medicine and American Heart Association amended their Physical Activity Guidelines and now recommend the following:

Guidelines for healthy adults under age 65

Do moderately intense cardio 30 minutes a day, five days a week
Or
Do vigorously intense cardio 20 minutes a day, 3 days a week
And
Do eight to 10 strength-training exercises, eight to 12 repetitions of each exercise twice a week.

Guidelines for adults over age 65
(or adults 50-64 with chronic conditions, such as arthritis)

Do moderately intense aerobic exercise 30 minutes a day, five days a week
Or
Do vigorously intense aerobic exercise 20 minutes a day, 3 days a week
And
Do eight to 10 strength-training exercises, 10-15 repetitions of each exercise twice to three times per week
And
If you are at risk of falling, perform balance exercises
And
Have a physical activity plan.

Both aerobic and muscle-strengthening activity is considered critical for healthy aging.  Notice how the current recommendations now include at least twice weekly strength exercises, as well as more vigorous cardiorespiratory exercise as an alternative to the standard moderate intensity activity to be performed most days of the week.

For more tips and information, please refer to the ACSM Physical Activity and Public Health Guidelines.