This warning can happing when you have a project that has the packaging type pom
and is configured as Java project in Eclipse. This also applies for or any other packaging type that is not supported by the m2e plugin.
In this case, the reason for this issue is that Eclipse validates the JDK version, but because of the packaging type m2e does not update the Eclipse settings when providing new source and target versions using the maven-compiler-plugin or compiler properties. The aggregator project usually doesn't need to be a Java project and might just accidentally got converted into one on creation or import.
The best solution would be to just remove the Java nature from the project. This shouldn't affect the build. In the Eclipse project explorer view, the "Maven Dependencies" will be gone though. It's possible that it could breaks some integration with some Eclipse plugins or features when the aggregator project is not a Java project any more. The modules of the project will still be Java projects.
To convert it to a non-Java project, you need to remove the Java project nature and Java builder from the .project
file in the aggregator projects root directory. You need to remove the line
<nature>org.eclipse.jdt.core.javanature</nature>
and the lines
<buildCommand>
<name>org.eclipse.jdt.core.javabuilder</name>
<arguments>
</arguments>
</buildCommand>
Safe the file and select the project in eclipse and hit F5 to update it.
Another solution is manually edit the Java version in the project settings. It could be that for some reason other target versions are defined somewhere that can't be set using th UI. For example I've found the entry org.eclipse.jdt.core.compiler.codegen.targetPlatform=1.4
in the .settings/org.eclipse.jdt.core.prefs
file which seems to have caused the warning even though every other setting was correct. If the warning still doesn't go away you might need to search in the setting files for similar entries.
A last option would be to add pom packinging type projects to the m2e lifecyclemapping, so that it will be converted to a Java project if it isn't already and updated when the target and source versions change.