Yes, 18-year-olds can attend high school in Japan, as typical senior-year high school students (Grade 12) reach the ages of 17 and 18 before graduating. Because Japanese academic calendars and birth-date cutoffs place students at specific ages during their final senior term, an 18-year-old completing their senior year is completely normal. However, if a student is significantly older or has experienced extended grade repetition past standard graduation windows, alternative adult education programs or night schools are typically recommended.