I have ADHD and I visit a pediatrician once a month but people say pediatricians are kid doctors does that mean when im 20 I can't see a pediatrician?

Eventually, yes. Pediatricians usually have a range of patient ages, and often transfer their care to other physicians when they age out. Of course this depends on the individual physician, so you should ask yours about this. You can also see an adolescent (or eventually an adult) psychiatrist for help with adhd.
Seeing a pediatrics. A pediatrician can treat oh until the age of 21.