We must always bear in mind that we should not imagine men living on in the spiritual world as they last were, when here. The trivial ideas that people hold, as, for instance, that those who die as children continue to live on as children, are naturally incorrect. The imagination may picture the dead as we last saw them here, but that is not their true form; it is rather the expression of it. A child may die, but the human entity incarnated in the child may be a highly evolved soul, and continue its life after death as a highly evolved soul.
Source: Rudolf Steiner – GA 157a – The Forming of Destiny and Life after Death – Lecture 3 – Berlin, 20th November 1915