Differences. I would look at the primary symptoms of social connection/disconnection. Autistic children are more difficulty to engage, establish eye contact, follow faces, learn familiar faces, and often don't coo or babble with their care givers. Adhd infants are usually quite social and interactive with their caregivers. In fact, because we give them so much attention, they may not even seem inattentive.
Autistic Spectrum. Disorder manifests as qualitative impairments in social communication & reciprocity, for which age-related lnorms are well-established. Such delays & deviations are harder to recognize than are poor quality of or delays in motor development. The average age for early-onset ADHD is 2 yrs.7 mos. Prenatal exposure to drugs, especially meth & alcohol can cause hyperactivity by 11-12 mos.