I live in colorado how do i go about getting a medical marijuana licence?

i'm 17 and i do have chronic back pain, and i already know smoking the ganja helps, so do i just tell my doctor that when i turn 18 and go from there or what? someone told me you have to be like 21 or something...anyone educated on this subject? also, if i move to another state that allows medical marijuana can i just transfer my med card or would i have to go through the whole process agsin?

: http://alcoholdrugstreatment.info/i-live-in-colorado-how-do-i-go-about-getting-a-medical-marijuana-licence