Intellij IDEA 13 uses Java 1.5 despite setting to 1.7

Syncing Maven or Gradle

When working with Maven or Gradle, it's vital to synchronize the compiler plugin settings with your IntelliJ configuration. For Maven, designate source and target to 1.7 in your pom.xml:

<plugin>
    <!-- This Maven doesn't fly without 1.7 -->
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-compiler-plugin</artifactId>
    <version>3.8.0</version>
    <configuration>
        <source>1.7</source>
        <target>1.7</target>
    </configuration>
</plugin>
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For Gradle, ensure your build.gradle reflects the correct sourceCompatibility and targetCompatibility:

// 1.7: The chosen One!
sourceCompatibility = '1.7'
targetCompatibility = '1.7'
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Update IntelliJ IDEA

Fight compatibility issues by staying current with the latest version of IntelliJ IDEA. Even though IDEA 13 is your current environment, it's worth considering upgrading to avail the newest language and tooling support.

Version control sanity check

Unintended reversions can occur as silly side-effects of using version control systems like Git. Stay alert for any commits that might have inadvertently tweaked the project/compiler settings.

Ensuring module consistency

Confirm the bytecode version of each module matches the project's settings. IntelliJ IDEA allows per-module configurations, which could provoke inconsistencies.

Visual guides aid

Intricate settings can be challenging to navigate. Keeping in line with up-to-date visual guides can be helpful. Check the IntelliJ IDEA documentation and relevant online resources for screenshots reflecting the current UI.

Don't hesitate to reach

Despite following the above steps, if the issue persists, consider reaching out to the community on forums, discussing the issue with colleagues, or consulting the proficient minds on StackOverflow.