Diagnose as early as possible but keep this in mind:
Its easy to diagnose a child. You dont have to be a psychologist to know that a totally introverted kid, with bad motor movements, and special interests has aspergers. In fact, i think many just totally get a wrong diagnosis and some even do google or self diagnosis thinking that 1, 2 or 3 symptoms such as a special interest, a bit anti social behavior, or no eye contact makes you a asperger.
I think the age of 12 is ideal (where all parts of the brain are already developed good enough such as formal thinking etc etc)
To be able diagnose a adult requires true talent. Now, its true that as an adult its harder to diagnose since they are already somehow adapted, sometimes just in a superficial way, and other times they really do become social, in this case you shouldnt get a diagnosis unless you think you are really helpless.
The diagnosis itself, if you think you have all symptoms, and really all of them or at least extreme difficulties reading facial expressions, disorientation, repetitive behavior patterns, then i say you really have autism, especially when you lack empathy.
Sometimes autism can be confused with borderline, ADHD or even sadism.
I just know one thing: I hate autism