Using your wording, the studies you are referring to are "still under question".  My point is that young adults (under 24) all over the world make good "adult" decisions every day.  Adults all over the world (over 24) make poor "adult" decisions every day.  I think child rearing and the child's willingness to conform plays a much bigger part then "frontal lobe development".

From my experience, individuals that focus only at the problem at hand without looking at other current or possible future problems tend to make bad choices regardless of age.