If I could answer, it might help to think about the existing aggravating factor that refers to young persons under the age of 18. While there will be differences for young persons, clearly developmentally there are some clear milestones for persons under that age. Most kids fall within that, though there is some variation.
The same can't be said for older persons. At a certain age chronologically, your capacity, vulnerability, is X, whereas based on a developmental capacity for children, you can come to some clear-cut understandings.
There is a distinction there. I think the question earlier was why the age is 18 for children. It does reflect some very clear and recognized capacity issues for persons under that age. You don't have the ability to draw the same line in the sand, categorically, for all persons at an older age.