My DD, age 12, 87 lbs is taking Adderall XR, 10mg. She just started Adderall XR and I believe that the dose is too low for her. Two years ago, she was taking Concerta 36 mg and I felt she could use more. The ADHD meds really do not "go" by weight, height, or age (unlike some medications that do). Some very lightweight people need an extremely large amount of meds to control their ADHD.
For instance, my son who is 8 years old has fairly severe ADHD. He is taking 20 mg of Ritalin LA (equivalent to taking 10 mg twice a day of regular Ritalin). This is a fairly low dose but it seems to control his ADHD. Any more Ritalin and he gets severely side effects.
My DD, described above, has borderline mild ADD (inattentive type). It seems to take a LOT of the meds to get an improvement out of her and she rarely exhibits side effects. She sleeps fine, doesn't lose her appetite, etc, until the meds get very high.
A lot of it has to do with how well you metabolize meds. Some people are ultra-metabolizers and require more, while others are poor-metabolizers and require much less because the liver and/or kidney does not break it down as quickly.
So, with the ADHD meds, you really can't go by age/weight/etc. But most doctors will start with the lowest dose and work upwards. Your are looking for maximum response with minimum side effects.