Japan Travel Fixes
What are the risks of hanging around an area like Shinjuku's "Toyoko"?
"Toyoko" has been repeatedly covered in the press as a gathering place for minors who have nowhere else to go, whether at home or school — approaching or engaging with them at all is an easy way to end up in trouble. The surrounding area is also known for illegal touts running rip-off bars that overcharge tourists.
Sexual contact with a minor, or any relationship involving payment, is a serious crime under Japan's Child Welfare Act and Child Prostitution/Child Pornography Act. "They consented" or "they said they were an adult" are not valid defenses.
The US, UK, Australia, Germany, and several other countries have extraterritorial jurisdiction laws allowing them to prosecute their own citizens for sexually exploiting a minor abroad — meaning even if no charges are filed in Japan, you could still face prosecution back home.
If you're worried about a minor who appears to be in trouble, Japan's nationwide child welfare consultation line, 189 ("Ichihayaku"), takes reports and concerns from third parties — it exists to help, not to accuse.