Can a child/adult with down syndrome be prescribed adderall or ritalin (methylphenidate)? if so would it have the same effect on those suffering from adhd?

Yes: While it appears that Down Syndrome patients are less likely to suffer from ADHD, medications used to treat ADHD are probably as effective in children with Down syndrome as they are in typical children.

Child. Psych: Should be seen to evaluate this question. Essentially looking to other reasons for poor attention or impulsivity hyperactivity problems eg. Psychosrimulants can worsen other conditions that may mimic ADHD - especially Anxiety, mood. And psychological tests for intelligence need to show that the child would be good candidate for the medication eg academic achievement and intelligence.