The correct dose of any stimulant medication is not based on age or body weight, but on how quickly an individual absorbs the medication into the bloodstream. Traditionally, one starts at a low dose. This is increased about once a week until it reaches an effective level.
It’s rare that the first dose is the right one for an individual with ADHD. Share this information with your family physician. (You might also show him my response to your question.) If this does not work, find a physician who is more experienced in treating ADHD.