I have had many chances to use the word "callow" lately, and I have found that, as is often the case with me, my use of the word was only in the penumbra of its definition. I had been under the impression that "callow" meant "unappreciative", "short sighted" or "insensitive". I think this was because I confused it with a similar sounding word, callous. "Callow" and "Callous" sound alike, and in general terms they refer to a person that may not understand the full value and meaning of things. However, the root words of the meaning are different. As Webster 1913 tells us, the root word of "callow" actually means "bald", and is cognate with Latin "Calvin". "Callous", on the other hand, is derived from a root word meaning "hard", and from which is also probably derived "Calcium" and "Calculus". While a "callow" person is metaphorically uncovered, a "callous" person is metaphorically too much covered. But in both cases, the result is a person who can overlook the value of things around them. Thus, two words with opposite meanings end up sounding the same, and meaning perhaps the same thing.
As an interesting parallel, or perhaps perpendicular to this, the Chinese language also has a very ancient word that is often translated "callow". It is also used as a hexagram in the I Ching, and can also be translated as foolish. However, the character involved "蒙", actually has a literal meaning opposite to the literal meaning of English "callow".
(And here I must interject a note that amongst sinologists, archaeologists and linguists, there is a great, unsolvable debate about how Chinese characters got their meanings. In this particular example, we have a pictogram of a house covered with moss or other epiphytes. Was this pictogram borrowed for a homophone, or was it extended to a concept with a related meaning? There is no answer to this question.)
The literal meaning of this character is "a house covered with epiphytes". In fact, that is what it is a picture of: a cottage with ferns growing off of the top. Metaphorically, it could mean someone whose senses and judgements are covered up. So in this case, the foolish young person is not seen as someone who is uncovered, but rather as someone who is covered up to the point where they can not detect the truth around them.