Curfews prevent important positive social behaviour and incubate a fearful mindset that increases anxiety and depression. Animals are happiest when they are free even if it means risk. Take a look at caged animals, they lose their shit. Locking people inside is a net negative experience and detrimental to a healthy society.
You could extend this logic to other things that are forbidden. Some people want to slap women's butts on the street (very common in Eastern Europe) but if we outlaw that are we treating people like caged animals and increasing anxiety and depression?
For the majority it cuts down legitimate activities and a healthy social life, though.
Smells a lot like older people who have their families and personal networks and have had their fun thinking that young people should stay inside and not be a bother. Thing is, young people need to socialize in order find a mate, build their social circle, establish their careers and engage with formative experiences. It's unjust for old people to decide they should be locked up after dark.