This may be true if you just mean calories, because elderly people usually do use less energy than they did when they were younger. But when it comes to other nutrients, the elderly need just as many as they ever did. All the systems in the body work together and depend on each other. When an ingredient is missing, the natural processes may falter. For example, infections occur more often and last longer in nutritionally deficient people than in well-nourished people because the body's ability to fight bacteria is weakened.
Variety is probably the most important ingredient.
No single food provides everything, so the best diet is a mix of dairy items, whole grains, fruits, vegetables, and protein. You don't have to eliminate favorite foods, just eat less of them. What really matters is the total diet consumed over several days. Here is an example of a balanced meal plan. The elderly often have a diminished sense of thirst and don't realize they need water.