Ex: The growth of alternative mental interests is a long process .
What the sentence means is that human beings in their youth, are primarily interested in "sense" experience. Youth easily understand the existence of the physical, because it is obvious and immanent.
[ See Synonyms for "immanent" as: built-in, constitutional, constitutive, essential, hardwired, inherent, inborn, inbred, indigenous, ingrain, ingrained (also engrained), innate, integral, intrinsic, native, natural ]
"Mental interests," or Mental Abstractions as they are evident in Thoughts, Concepts,
and Ideas seldom appear as a vital reality or a meaningful reality until people are older.
Thus, the reference to a "long process". As people mature, they come to realize that "sensory" experience is the lesser part where the discovery of meaning and purpose in Life is concerned.
As human beings mature, they begin to pay more attention to the Mental Abstractions which are associated with Philosophy, Science, Law and Religion. Morality in particular, which pertains to both Philosophy and Religion (not Science) becomes a subject of great interest. In such endeavor, questions about Mind, God, a Transcendental SELF or a Soul or Spirit become subjects which assume a greater importance.
"Mental Interests" can be expressed by discussion, and more reading, and the exploration of ideas which were not deemed important when "interests" generally were the pursuit of youth.