Most conclusive studies on developmental psychology and neurobiology indicate that the brain does not begin to enter the later stages of growth (in which one could be considered a fully rational cognizant human being) until at least 25.
The prefrontal cortex is the area of the brain that is associated with response time, decision making, sequencing, precision and executive functions. Which are all items measured when testing for cognitive capacity. An 18 year old scores much, much, much lower than someone even 23 or 24. It's an entirely irrational system.
The complete jumps in the standard legal age for adulthood over the last, even a hundred years, definitely support my statement. I have no doubt that as we learn more about the brain, that legal age will end up changing in the future as well.