@EroZero Teenagers will have sex. Eventually your kids will have sex. They will sneak off experiment. It is part of life. This is why we as their guardians and parents advice them to use protection. Trust your instincts. If you aren't sure, come to us for a little wisdom. Or someone you can trust.
Reminder that, at 18, she's still technically a "child" in Japan, who come of age and are considered adults at 20. I do believe she's still not allowed to marry without parental permission.