I believe there's something arbitrary about every decision with regard to age; that's obvious.
At one point, I believe most people who are concerned with the development of human beings have shown that 18 is the average age at which a larger number of individuals are slightly more responsible than at a younger age. You ask me whether there are any 19-year-olds who are less responsible than 17-year-olds. I would say we have to consider that on an individual basis, when we talk about needs.
That said, an age had to be established in the act. If you visited a penitentiary tomorrow, you would find adults who are still irresponsible, but they may constitute a minority.