Age limits need to be defined for legal purposes - yes 18 plus are treated as adults by medical services (as an example) so parents of a 17, almost 18 yo are automatically expected to be involved. A few days, weeks, months later, when they are 18, this isn't the case unless the 18 yo wants it to be. Doesn't mean the 18 year old is suddenly totally grown up or conversely that 17 year olds aren't capable of dealing with medical services by themselves either.
At work we have a 26 year old who could pass as a teen some times in terms of their levels of responsibility and maturity. Then another 20 something who has the manner and capability of the average 30 plus adult.
People are individuals - whatever their age. Snap judgements based simply on their date of birth should not be made.