Is The Clockwork Orange mature or immature?
Yes, there's lots of phallic imagery, juvenile humor, slapstick, gore, and all sorts of things that appeal to immature teenagers.
But with rape, murder, prison, gang violence and so on, these are all adult experiences. People have vices, mental disorders, unfulfilled needs and temptations. Politics and criminal justice are often controversial topics, but they're adult experiences nonetheless.
When we have freedom, we have choices. We can choose good things. We can choose bad things. What's immature is living in a Peter Pan fantasy world where people never face real-world issues and only Disney movies are wholesome enough.