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 respond on the mobility of an element. If elements are actively mobilised, then younger parts of plant show deficiency symptoms if they are not transported to older parts/mature parts of plants. Deficiency symptoms of N, K, Mg are first observed in senescent leaves. In older leaves, the biomolecules are split into elements and it makes them mobilise into younger leaves