How likely is it for a child to grow out of adhd?

Does happen. A good 30% of kids loose the features of this syndrome by adulthood, another subset of kids decide they can manage the symptoms on their own & choose not to deal with the alteration in their thinking/side effects of meds after adolescence. Some will benefit from treatment as adults.
Some do,some don't. Approx 3 out of 10 no longer exhibit significant symptoms, or those symptoms do not cause significant impairement by late adolecense or adulthood.As compliance and adherence to medical intervention decreases through out the years;the important question is what is been done in addition to medication to improve outcome.An adhd program-medication, behavioral intervention, skill training etc, is key.
Less than 50% Less than 50% grow out by end of high school. A significant percentage learn how to manage their symptoms in adulthood and may not rely on medication.