Infantilization culture (examples of Infantilization)

Once a person turns 18, they are considered by their culture as an adult. But they're still treated like children, despite their age and what that age symbolizes and represents to society. As and adult, certain privileges are granted, like the ability to vote, purchase cigarettes and pornography.

If you're old enough to smoke and buy porno, then why must infantilization continue occur? Some examples include a 18 year old student asking a teacher "may I go to the bathroom?" and wait for the teacher's answer, when we can just as easily say "hey, I'm going to the bathroom, I'll be right back." and teachers or other adults talking down to us like they would talk to a baby, and the worst..bedtimes. If I'm old enough to smoke and purchase items of adult content, and vote! then why should there be a set bedtime, I'll go to bed whenever I feel like it, if one is old enough to smoke or vote, then why can't that person choose the very time that they go to bed? Which leads to an all more important question: if I can vote, then why can't I make decisions for myself?