While real Japanese teenagers do occasionally live alone, it doesn't happen nearly as often as is suggested by anime. Generally, a Japanese student will only live alone if they are attending a prestigious high school that's far from their family's home. A student's school is not assigned based on their home's location, so many children face long daily commutes. Additionally, most Japanese high schools do not have dormitories, so it's possible that a student might need a place of their own.
In contrast to reality, there are a surprisingly large number of anime teenagers who live alone, or who share a house with other minors. Sometimes, the student isn't even in high school; Ryou Machiko is a middle school student who takes over her family's home after the death of her parents.
As long as these anime teens can support themselves, it's not technically irresponsible to let them live alone, right?
One of the most disturbing things about Naruto is the village's utter lack of a system for taking care of orphaned children. Naruto is 12 years old when the series starts, yet he lives completely on his own, as his parents died trying to save their village. Despite being the son of the village's former leader, Naruto has basically been on his own since he was an infant.
Naruto's isolation is perpetuated by the village's fear of his hidden power. A nine-tailed fox beast is sealed within his body, and there's no telling when it might choose to reemerge and wreak havoc on the town. However, Naruto is far from the only child who lives alone, which suggests that the society lacks any sory of foster care system. After an adolescent Sasuke's entire clan is slaughtered, he ends up living alone on his family's estate, as none of the village's adults even consider adopting him.
Orihime Inoue of Bleach lives apart from her parents, and for good reason. When she was three years old, her older brother kidnapped her in order to protect her from their abusive parents. After that, he looked after her, up until he died in a car accident when she was 12.
Since the tragedy of her brother's death was a public spectacle, one might think that a social worker would step in to look out for the young Orihime, but instead she begins living on her own. Her aunt agrees to support her financially as long as she keeps her grades up, but a good parent provides more than just financial support.
It's pretty normal for children to live on their own in the world of Pokémon. The very premise of the show is that 10-year-olds regularly embark on multi-year journeys wherein they regularly encounter dangerous monsters.
While the heroes of Pokemon are uniformly independent, most of them still have a family home to return to. Brock is the notable exception, as both his parents left when he was 15 years old to pursue their own Pokémon journeys. When this happened, Brock was left to take care of the family gym, as well as his nine younger siblings. While Brock's parents do eventually return to lend the poor teen a hand, asking a 15-year-old to step up and become the head of a 10-person household is still pretty ridiculous.
Shino Asada — Sword Art Online II
Sword Art Online II's Shino Asada lives on her own so that she can be near her high school. While originally, Shino's school was very close to her home, a freak occurrance forced her to transfer to a school far from where her family lives.
After Shino killed a bank robber to protect herself and her mother, she faced a lot of harassment regarding the incident when she returned to her original school. Unfortunately, the transfer fails to stop the bullying, and actually puts Shino in a dangerous situation. Once Shino begins living alone, she becomes an easy target for a violent criminal who has it out for her.