The quality, state, or characteristic of being boyish; youthful, playful, or immature qualities associated with boys.
From boyish combined with the suffix -ness (which forms abstract nouns describing qualities or states). The word captures the essence of boyish character rather than the action of being boyish.
Boyishness is interesting because it's one of the few qualities that can be endearing in a 10-year-old and ridiculous in a 40-year-old—context completely changes what the word means.
Complete word intelligence in one call. Free tier — 50 lookups/day.