It varies. Most children with autism begin to show signs around the age of two. It can take many years beyond that to obtain an official diagnosis, however, especially if the parents are unaware of the symptoms and the child speaks on time. It also depends on where you live and your access to specialists who can accurately make the diagnosis.

Most children are diagnosed at the age of 2-2 1/2 that's when most psychologists say that it's easier to tell because of the mile stones are clearer at that age.  I personally knew something was going on before my son was diagnosed at the age of 2.  He's 4 now, and is still unable to speak, although we have developed other means of communication.