Theres a curfew here where i live. its for anyone under 18 where after 11pm they are not supposed to be out an about. It is rarely enforced, on a city wide level when the 2 local highschools play games against each other or something like that is usualy about it and then its announced in the paper that it will be strictly enforced, no questions asked. the rest of the time its pretty much just if the kids are acting up or looking very suspicious that they will enforce it.
There are exceptions like when the schools have a dance where the curfew is pushed back to 1 am for students that attended, we have a couple 24 hour restraunts that some go to after the dance. Also if you go to the local movie theater if the movie ends after 11 then its 2 extra hours from when the movie ended, again so you can grab food on the way home. but in either case you will need proof if a police officer decides to question you.
Prior to the curfew there was more vandalism and petty burgalry and cars being broke into. so it does help even if it restricts the teens.