First you have to think about what makes up a balanced diet: fruits/vegetables, protein, whole grains/carbohydrates, and fat.
At every age the body needs vitamins and minerals, which will most likely come from fruits and vegetables. In fact, experts now believe (and vegans and vegetarians agree) that fruits and vegetables supplemented with legumes and nuts can almost singly provide every nutrient the body needs.
Fat, protein and iron are especially important in really early years (under the age of 3) for brain development, which is why doctors will tell women to switch to formula (if child is under one) and then whole milk after breastfeeding.
Calcium and vitamin D (which aids the absorbtion of calcium) is most important during growth and bone development (up to about age 21 in males and about 18 in females), but then again it becomes important after the 40s with the risk of osteoporosis. Much more prominent in women, osteoporosis also affects men.
Whole grains/carbohydrates are necessary to every age - but should be balanced by activity in addition to other nutrients on the food pyramid. Many experts are beginning to say that the original food pyramid could have fat cut off the very top and be turned upside down - because it is actually healthier to consume more fruits and vegetables than bread and grains. Bread and grains contribute to complications such as diabetes - because we are so used to eating white flower, white sugar, and adding gluten to nearly all of our processed foods. The fact is, again, many vegetables contain the right amount of carbohydrates our bodies need for energy - and taking in more sugar than our body burns results in the storing of excess fat.
Finally, check out the article below. It has more information on specific nutrient advice for the 20s, 30s, 40s, 50s and 60s.
The nourishment of the various systems of the body is something that is only vaguely understood, and though the current understanding of it is based on reductionist ideas (stripping everything down to what we think are essential nutrients, etc.) there are any number of important connections between various nutrients and the bodily systems they support.
If you choose one time in the life cycle, perhaps adolescence, there are any number of intense physical and chemical changes going on within the body, and to support them healthily, one must be ingesting the various foods and their nutrients necessary for those changes and processes to proceed smoothly.
It is important for people of all ages to have a balanced diet. Dietary needs are different for everyone depending age, gender, physical condition, etc. Food sources come from the milk group, meat and beans group, fruit group, vegatable group, grain group, and oil group. When people do not consume the right amount of foods in each group they may experience problems with growth and development and brain and nervous system function.
Everyone needs a balanced diet but I will focus on elderly people because they have some very specific dietary needs. Elderly people not only need to have a balanced diet but they should also take some other very important things into consideration. For example, elderly people need to consume extra water, extra protein in order to maintain a healthy immmune system, and consume higher amounts of fiber. Fat intake should also be limited. Foods containing iron, zinc, B12, and calcium should also eaten regularly.