The Alfresco cm:created property is a protected field. It normally can't be changed.
But query the property again or look at the value in the Node Browser and you'll find that it hasn't changed.
Sometimes it's desirable to be able to change the created date of the node. This is especially useful when content is being migrated from another source into Alfresco and the creation date from the other system needs to be preserved in Alfresco.
Create and compile the Java file com/formtek/example/util/RunAsSystem.java:
Then wire this file into Alfresco with the file alfresco/extension/runas-context.xml:
In this example, it changes the creation dates on one of the standard sample documents that comes with Alfresco.