A language doesn't just keep getting better and better with age. "Maturity" is about achieving a a level of stability and quality in language features, tooling, runtime, library ecosystem, etc. Scala and Java share the same runtime and library ecosystem, and Scala is arguably more mature in its language features, since Java has been forced to add features (and incur extra complexity) playing catch-up to newer JVM languages (including Scala.)

Both languages actually suffer from maturity in their tooling, because their standard build and dependency management tools (maven and sbt) are outdated and crufty, while newer languages such as Go and Rust have tooling that was built more recently, with the benefit of more recent experience.