Do legally recognized adults over 18Y lack the capacity to make their own decisions

Many colleges still ask on students bringing their parents even for small issues. This happens even when students are 22 years old. If an 18-year-old is considered incapable of making their own decisions, then why does the government treat anyone above 18 as an adult? 18 vayass olla oruthanu swantham ayi therumanich vote cheyyan olla avakasam und pakshe swantham karyam therumanikkanm engi vere arelum venm