Java^[1], 61 bytes

This is valid in both Java 5 and Java 6. This won't work in Java 4 or earlier (because enum didn't exist) and will not work in Java 7 or after (because the bypass^[2] was "fixed").

1. As of today, this is still the shortest Java answer in all versions of the language. So this is why it's not named "Java 5 & 6" or equivalent. When shorter answers will come up (and they will come up because some JEPs intend to shorten what it means to be a Java program), then only it will be relevant to say Java 5 & 6.

2. The bypass consists of the static execution of code when the class is being linked. Before Java 7, the main-method-containing class was no exception to the static initialization code. Afterwards, static initialization was delayed until the main method would actually be found.

Java 5, 61 bytes

This is valid in both Java 5 and Java 6. This won't work in Java 4 or earlier (because enum didn't exist) and will not work in Java 7 or after (because this solution uses a bypass^[1] that was "fixed").

1. The bypass consists of the static execution of code when the class is being linked. Before Java 7, the main-method-containing class was no exception to the static initialization code. Afterwards, static initialization was delayed until the main method would actually be found.

Java, 61 bytes

This is valid in both Java 5 and Java 6. This won't work in Java 4 or earlier (because enum didn't exist) and will not work in Java 7 or after (because the bypass^[1] was "fixed").

1: The bypass consists of the static execution of code when the class is being linked. Before Java 7, the main-method-containing class was no exception to the static initialization code. Afterwards, static initialization was delayed until the main method would actually be found.

Java^[1], 61 bytes

This is valid in both Java 5 and Java 6. This won't work in Java 4 or earlier (because enum didn't exist) and will not work in Java 7 or after (because the bypass^[2] was "fixed").

1. As of today, this is still the shortest Java answer in all versions of the language. So this is why it's not named "Java 5 & 6" or equivalent. When shorter answers will come up (and they will come up because some JEPs intend to shorten what it means to be a Java program), then only it will be relevant to say Java 5 & 6.

2. The bypass consists of the static execution of code when the class is being linked. Before Java 7, the main-method-containing class was no exception to the static initialization code. Afterwards, static initialization was delayed until the main method would actually be found.

Java 5 & 6, 61 bytes

Java, 61 bytes