Besides having far more psychotic affects, pot stays in the bloodstream for over a month, so the brain never clears up if the frequency of use is less than that. Alcohol is out of the body in a few hours.
California Law Enforcement Forum on College Drinking
Many factors may explain colleges' failure to implement the NIAAA's recommendations to control underage drinking on campus: a lack of university funding, a lack of time, a perceived lack of authority or jurisdiction within the community, or even a lack of interest on the part of the university, many universities even see the program as a waste of resources. Whatever the reasons may be, a multitude of options are available should colleges choose to institute programs to decrease instances of underage drinking on campus. These options include, but are not limited to, alcohol education programs, social norms campaigns, substance-free housing, individual interventions, parental notification policies, disciplinary procedures for alcohol-related violations, and amnesty policies to protect the health and safety of students. 
Even young adults want to keep the age as is: only 24% of those 18-29 want to lower the age. Similar results were found in 2001 and 2007.
In a 2002 meta-study of the legal drinking age and health and social problems, 72% of the studies found no statistically significant relationship despite claims that lowering the MLDA to 18 would increase suicide and criminal activities by adolescents. [ 19 ]
In a state of alcohol impairment (short-term effects), the adolescent brain is impaired in ways similar to the adult brain but to differing degrees. In various animal experiments, the adolescent brain shows greater alcohol-induced impairment of memory retrieval and imprinting capabilities than the adult brain. This is to say that the adolescent brain is worse at remembering things while drunk, and remembering things that happened while drunk. Contrastingly, the adolescent brain is less impaired in motor skill control than the adult brain. That is, the adolescent brain maintains a higher level of balance, reaction time, and hand eye coordination while under the same level of impairment as an adult brain. Since it is unethical to supply alcohol to those below the drinking age in a laboratory setting, researchers are forced to use rats for experimental trials. Any application of these findings to humans is necessarily somewhat speculative.