The importance of maintaining good flexibility as we age provides many benefits. Specifically, it will reduce our chance of injury and pain as we age. Maintaining flexibility will help us avoid such as classic symptoms such as knee pain and low back pain.
Flexible muscles are much more responsive than inflexible muscles. They allow us to absorb shock better. This will help to reduce the stress and impact on joints, especially during activity.
Stretching post-exercise is beneficial because the muscles are still warm from exertion which allows for a better stretch and decreases the chance of injury. Stretching after exercise also helps with recovery and will help with any post-exercise soreness.
I suggest stretching 5 to 10 minutes for 3 to 5 times a week. This is sufficient to improve and maintain good flexibility.