When it comes to buying marijuana from a dispensary, you need different kinds of document depending on whether you are buying medicinal or recreational weed. For recreational weed you need an age proof which shows you are 21 years or above (the age limit may vary based on your location). However, when it comes to buying medicinal marijuana, you need a medical cannabis registry card from your department of public health. Even for this there is an age limit, which is usually over the age of 18.