In the simplest of definition, a man child is an immature guy who just refuses to grow up. This doesn't mean he necessarily lives at home with mom and dad ― although he probably should considering the way he functions in the real world ― but that he just doesn't have his life together.
A man by age but a child in mentality and actions. Usually an inability to get over the "frat boy" or "party boy" phase. Can't have fun without a beer; oftentimes brews his own beer. Still relate to women as sex objects (even in they're married) and are emotionally undeveloped (e.g., fight for their rights mentality even if there is nothing worth fighting; rarely takes responsibility for own failings). A general irritant to women and men that have matured.
Facebooking about brewing beer and gross bodily functions is typical of a man child.
A man child is a male who is over the age of 25, who still lives as though he is a juvenile. Many man-children are not gainfully employed, and survive off of the financial support of their enabling parents. Heavy video-game addiction to fantasy games such as World of Warcraft are key to the man-child.
"You'll never believe it, but that 30-year old uber-gamer man child who lives door to me finally got a job!"
A man child is an individual (usually male) who still hold traits of a child (adolescent). These traits include whining, pettiness, insecurity and not being able to take the blame.