How Old Do I Need to Be to Get a Missouri MMJ Card? Patients must be aged 18 years old or over in order to qualify for a medical marijuana card and physician’s certificate for themselves. Caregivers must be aged 21 or over. Those under 18 must have a designated primary caregiver for medical cannabis and cannabinoid-based medications (although this may not necessarily be the case for the FDA-approved Epidiolex!).