When you stop reading and start answering questions in Stack Overflow.
When you start consulting the official manual of the language to check for difficult to interpret programming details.
When you contribute to the community
When your co-workers use you as a reference for the language
When you start seeing beauty (and ugliness) in programs
When you start disliking the code you did some time ago, because now you know more than that
When your errors become much more complex, because you don´t make simple errors anymore
When even without thinking your code is consistent, and you follow the same programming patterns and they are all clearly good.