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

(N/A) Deficiency symptoms are morphological changes in plants that indicate nutrient deficiency. The appearance of these symptoms depends on the mobility of the deficient element within the plant.
$1$. Elements that are actively mobilized: Elements such as $N$,$K$,and $Mg$ are highly mobile. They are transported from mature (older) tissues to younger,developing parts of the plant. Consequently,deficiency symptoms for these elements appear first in the older parts.
$2$. Elements that are relatively immobile: Elements such as $Ca$ and $S$ are structural components and are not easily transported out of mature tissues. Therefore,when these elements are deficient,the younger parts of the plant,which are actively growing,show the symptoms first.