Answer: The juvenile system is for minors and the adult legal system is for adults.
Explanation: The juvenile was established to pass judgments for crimes that are committed by minors (anyone under 18). The adult legal system passes judgment on crimes committed by adults (anyone over 18).
For a more specific answer: adults are charged with crimes, juveniles are charged with delinquent acts (unless they've committed something as serious as murder, in which case they'll be charged as an adult).