O.Henry uses the word in an unusual way. I wondered what
the etymology might be. The definition given by AHD doesn't jibe with the definition, which in one word is "immature"
But the etymology is from a word meaning "bald". Well, absence of testosterone may delay scalp hair loss. The chippy chippy chop on a big black block between the thighs
is a high price to pay for a head of hair. I have never seen, and do not expect ever to see a bald callow youth.

"The callow grass between the walks was the color of verdigris, a poisonous green, reminiscent of the horde of derelict humans that had breathed upon the soil during the summer and autumn."

callow

SYLLABICATION: cal·low
PRONUNCIATION: kl
ADJECTIVE: Lacking adult maturity or experience; immature: a callow young man.
ETYMOLOGY: Middle English calwe, bald, from Old English calu.
OTHER FORMS: callow·ness —NOUN