For what reasons don't doctors diagnose autism before age 3?

Sometimes they do. It should be able to be recognized by age 3, but if symptoms are less noticeable, and iq is high, the diagnosis may not always be determined by age 3. In some cases the diagnosis is quite obvious to an experienced clinician by age 2. Feel free to seek a second opinion. Don't let the lack of a diagnose keep you from getting your child help. Speech/occupational therapy services can be very helpful.