By default maven sets the compiler version for you. Of course, you can always set it in the pom, but there are cases where you cannot modify the pom, and/or you might want to test compilation and tests with different versions of java.
Following are the specific arguments to pass the compiler version to maven from the command line:
mvn clean install -Dmaven.compiler.source=1.7 -Dmaven.compiler.target=1.7