When can autism be diagnosed?

Very. There is no "official" age at which the diagnosis can be formulated, however, some of the criteria is only observable (or the lack thereof) after the kid is supposed to reach a milestone (verbal skills, etc). Review the criteria dsm4 and you will have a clear idea of why. That said, some genetic diseases ( deletions, isodicentrism, etc) associated with autistic features can be diagnosed at any age.