The statute of limitations for an Alabama prosecution on a drug possession case is three years from the arrest, normally. If you have been charged after that period, you would have a statute of limitations defense to this charge.
Without conferring with the DA or seeing the court papers, I could not tell you why the state feels that they can still prosecute. What I can tell you is that they believe they still can and that they would be prepared to make an argument to that effect.
If you've been charged with trafficking, there is no statute of limitations for that offense. So they are free to arrest you any time at all and the case would never be too old to be prosecuted. There are also other circumstances which could override the statute of limitations, for example, if you'd warranted on the case or if you'd fled the state.
So if you have been arrested now, you should should hire counsel. When you show up for your arraignment, you could plead not guilty and you charge was not a trafficking drug charge your lawyer would ask for the case against you to be dismissed because of the statute of limitations.
The DA would then make an argument as to why that statute does not apply. Your lawyer would either make a counter argument or ask for a continuance to research the issue and file a memorandum of law in support of your position. The judge would then rule. If you win, the case would be dismissed and would be too old to be prosecuted. If you lose, the case would go forward and whether or not the judge was correct about that would be a matter for appeal if you were convicted later of the charge.