Why are children and elderly the weakest forms?

Lets put it this way:

When you are a child, your body is just developing. You may not be as consious of your health as adults will be.

When you become an adult, you have reahced the peak of your development. Your are now fully aware of your health and can learn to look after it.

When you reach the elderly age, your body systems become worn out and weaken from all those years of work and play. Therefore you don't have enough energy and strenght left to fight the infection or disease.