In this recent study from Canada, the peak ages for starting methylphenidate seemed to be at the start of school.
What was surprising was the high rate of its discontinuance by 2 years.
In this figure, the cessation rates are plotted against the age of children when the medication was first begun. The very high discontinuance rates for both boys and girls at age 2-3 raises the possibility that some physicians have jumped the gun with prescriptions, treating normal developmental variations in activity and temperament with stimulants because of suspected ADD or ADHD.
This is concerning particularly as other studies have shown dramatic increases in the prescription of psychotropic medications for preschoolers. In the early 1990's, increases of 3-fold were seen. This figure is almost certainly higher today.
Starting and Stopping Methylphenidate
Preschoolers and Psychotropics
Controversy Within FDA Re: Black Box Warnings - Psychosis