I don't have a particularly strong feeling one way or another about the age of the killer.
I will say, that as a child/teen our 'gang' of neighborhood kids (wide range of ages) lived near a creek and woods. When you wanted to do something bad, the woods was the place we always went. As a kid/teen, you don't have much privacy or a place of your own (bedrooms are usually within earshot of parents) and the woods offered that privacy from adults.
Jenise being put in the woods does remind me of something a teen might do. Many posters have said if it was an adult with access to a vehicle, they would have driven her away and disposed of her elsewhere. I don't disagree with that idea, as it would decrease chances of Jenise being found. But another reason an adult would not drive Jenise away is if he did not want to risk her DNA/other evidence getting in his car.
I'm on the fence.