Missouri law dictates you must be 21 years of age or older to purchase, possess, transport, and use cannabis. So, if you're not 21 yet — hang tight! Medical marijuana patients must be 18 years of age or older. However, they must also have a valid doctor’s recommendation and medical ID card.nIn California, the legal age to purchase, possess, and recreationally consume cannabis is 21 or older. If you're 18 or older and have a valid medical recommendation, you can purchase cannabis from any licensed dispensary with a valid government-issued photo ID.