(Australian context) A boy who goes through puberty at a very young age, and is usually the tallest and oldest looking amongst his peers between the ages of 11-13. After that, he stops growing, and his peers go through puberty and become the same size/taller than him in the later teenage years. Despite this, however, a manchild will retain his characteristically mature features well into later life.
twelve year old boy1: dude, that guy over there has already started shaving. And he's like taller than all of us.
twelve-year old boy2: Yeah, he's such a man child!
(4 years later)
16-year old boy1: Hey look, it's the same guy we were calling a manchild 4 years ago! What happened man, he used to be taller than all of us, now he's like 5'9.5'' and we're all 6'1'' !!
16-year old boy2: I know, he's actually the same size he was when he was 12. But dude, he's still a manchild- look at his awesome stubble and hairy arms!