You absolutely can be diagnosed when you are older. I was working with a therapist on my severe depression and anxiety disorder (just a couple of a long list of problems) and one day he says, "You ever hear of Asperger's?",and we just went on from there and a short time later I got the 'official diagnosis' (whatever that means). Wasn't really a surprise though given my history but it did help a lot of things fall into place.