Not uncommon to have 15 year old parents and soldiers in some parts of the world today - sadly.
The issue is, for a psychologist, whether impulse control is sufficient at a certain cut off age. For those who want to lower the age to 18, the strongest argument is the rights arguments based on voting, marriage and entering the service.
If you want to argue the neurophysiology of impulse control that is not the best argument for the lowering of the age, I have to say. It has been argued that in good old days, the younger males were supervised by older males more than today.
Sorry to be a downer on that, but I know what I know.