A person is an adult for criminal justice purposes at age 17 in Texas. Turning 18 doesn't do anything. If the charges happened at age 17 or later there will be a warrant. If the charges happened before he turned 17 there will be a directive to apprehend.
I'm going to disagree with Mr. Daniel a bit. Actually, with pending charges that occurred when the person was 16, the age of 17 doesn't matter, but 18 matters a lot. If he hasn't been adjudicated on the misdemeanor when he turns 18, they close the case. If they somehow let it fall through the cracks and he gets arrested on the warrant after he is 18, then he'll be held until it comes to the court's attention and the DA's office does the paperwork to close it. It's better, though, to get him turned in. Because it's a misdemeanor, he won't ever be sent to TYC (TJJD) on it, and he will be able to get his record sealed as long as there isn't a felony at any time. But the juvenile system might be able to at least try to rehabilitate him. Running from the law at his age really doesn't do anything but get him into bigger trouble out on the streets.
You really need to contact a lawyer. It is unclear what age the juvenile was when he was charged with possession. In GA the statute of limitations runs if the case hasn't been accused, do you know if the case was accused? There are many facts an attorney needs to know that simply can't be answered by an on-line general answer.