I'm from Colorado so this issue has been a hot topic while it was getting legalized.
My opinion is that it should be left to the states to legalize.
A lot of what I have read show that using it at a young age can impare mental development. After you have fully developed (after 21), the consequences are not as extreme and I would consider it similar to alcohol. If you use after 21, its not a big deal other than it makes you unproductive and lazy... similar to how drinking a lot the night before and being hungover would make you worthless. If thats what you want to do, have fun.
If it is legalized, treat it similar to alcohol. 21 year old age limit, driving restrictions, and your work has reason to fire you if they choose to perform drug testing. Exceptions are for medical purposes, where it has been shown to really help with children you experience severe seizures.