Admitting to using or possessing drugs after the statute of limitations has passed means you cannot be charged or convicted with anything. So there is no legal harm in the admission. Thought it might make the police curious as to whether you are still doing drugs and could bring undue attention to you.
Just admitting you did drugs in the past, even when the statute of limitations has not yet passed, is not likely to be enough to convict you without further proof, but it would be unwise to confess such a thing.
The social ramifications of admitting you did drugs in the past would be another issue.