In the US you graduate high school around 18 and then get a 4 year bachelor's degree after which you apply to medical schools (which are an additional 4 years). After medical school you graduate as a doctor but before you are allowed to practice on your own you must complete a "residency" in order to be fully licensed. This is known as specialization also. A residency can last anywhere from 3 to 9 years depending on the field you go into (internal medicine being the shortest, different surgical specialties being the longest). After residency you are a fully certified physician in the US.