If you're being juvenile, you're not acting your age. Unless of course you are a juvenile. In which case, carry on.
While the noun juvenile refers to someone not physically or emotionally mature, the word is often used as an adjective to refer to anything specifically for young people, such as juvenile reading materials or movies. It can be derisive, as in "Don't act so juvenile," suggesting a silliness unbecoming an adult. The word comes from the Latin juvenīlis, meaning "youthful" — similar, but much more flattering.