Curfew

At least Protected content cities have curfews on teenage youth, including 78 of the 92 cities with a population greater than 180,000. In most of these cities, curfews prohibit children under 18 from being on the streets after 11:00 pm during the week and after midnight on weekends.
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I had always imagined that in a common law country like the USA there was a presumption of innocence until proven guilty so I'm surprised that teenagers have their liberty restricted in this way. I don't want to offend anyone but I'm curious. Is there any evidence that crime is reduced by this legislation? Is it constitutional? What do the teenagers themselves think? Is it imposed only on those who have no right to vote? Do any other countries have similar laws? What about homeless people? What about night workers? :)