It's so hard to view Mugi is someone right out of high school. Either he had an abnormal, that is, challenging, childhood or he's a bit older. Since we do know that's his age, it's the setting, it automatically means he must have gone through a lot growing up. It doesn't necessarily mean he needed to experience violence and such, maybe he just had nobody to rely on and had to take care of himself from early on, making him grow up much earlier than kids normally do.