What age groups suffer the most from malnutrition?

In general:

Children, the elderly, and those suffering from disease are most at risk for malnutrition. People with higher nutrient needs (like children or pregnant/lactating women) may be at risk for malnutrition if they do not consume the appropriate nutrients.

Those suffering from/living with disease often have higher nutrient needs because their bodies are under a lot of stress, but may not be consuming enough to make up for their incredible needs.