Why is that in certain plants deficiency symptoms appear first in younger parts of the plant while in others they do so in mature organs?

Deficiency symptoms are morphological changes in plants, indicating a nutrient deficiency. Deficiency symptoms vary from one element to another. The plant part in which a deficiency symptom occurs depends on the mobility of the deficient element in the plant. Elements such as nitrogen, potassium, and magnesium are highly mobile. These elements move from the mature organs to the younger parts of a plant. Therefore, the symptoms of the deficiencies of these elements first appear in the older parts of the plant. Elements such as calcium and sulphur are relatively immobile. These elements are not transported out of the older parts of a plant. Therefore, the symptoms of the deficiencies of these elements first appear in the younger parts of the plant.