(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!
A man that is so large and/or powerful that he makes other men around him look like children. Usually an athlete that dominates others on the field/court because of his size and strength.
An boy aged between 12 and 16 whose face has outgrown his body during adolescence, resulting in the awkward imbalance of having a man's face on a boy's body. Man children are invariably no taller than 5'7'' and are sometimes (not exclusively) afflicted with severe acne which extends to the chest and back. Man children are usually quiet and shy in social situations.
Person 1: "hey quick look across the road there's a man child, a real goodin"
Person 2: "Mate, wicked man child spot! Why do you reckon they always wear cargo shorts?"