Well if it ever were legalized it would be for people over 18 anyway, possibly even 21. So i don't really see how this is an issue.
in other news, weed has been found to:
-provide effective pain relief
-Manage ADHD
-Reduce Anxiety
-Increase Empathy
-Help safeguard certain portions of the brain against aging and help improve cognitive function
-Increases in lung air flow rates and increases in lung capacity
-Effectively treat multiple sclerosis, Tourette syndrome, obsessive-compulsive disorder, brachial plexus neuropathies, insomnia, memory disorders, anxiety disorders, cancer, neurodegenerative disease
The list goes on and on and on.
Obviously its not perfect and has some negative effects such as the one proposed here, but realistically, if someone of age is using the plant, its doing far more good than harm. The worst thing that will probably happen is you'll eat all your snacks or waste too much time playing video games when you should be doing work/something more productive.