The title explicitly states the premise: a narrative turning point where a male protagonist (shounen) crosses the threshold into adulthood (otona) during the specific, nostalgic season of summer—often associated in Japanese media with freedom, growth, and secret encounters.
“Oi, Kaito! You gonna stare at bugs all day or help us build the raft?”
“You’re... here.”
“Let’s do it again.”
The phrase "Shounen ga Otona ni Natta Natsu" translates to The Summer the Boy Became an Adult