I think it can be developed at any age, depending on the environment. I do believe that most cases of autism are developed at birth, then just diagnosed later in life. The environment of the person is one of the biggest factors, although genetics also play a heavy role in it.
Symptoms of spectrum disorders can appear at a range of ages in childhood. While most kids show autistic behavior before age 3, it is possible that you may not notice it or get a diagnosis until 5 or later.
Yes, in fact, the age of five is usually when signs of autism start to show up (lack of speech, eye contact, social skills), etc. Signs generally start at 3, but by 5, you will probably know.
Autism rarely affects children over the age of 3, however there is always a chance that any age can develop this life long incapacitating life long developmental disorder. Children who do have this disorder can be as normal as any other child.
Autism generally appears in the first three years of life. Most parents suspect that something is wrong with the child around the time the child is 18 months old and then seek help for the child sometime after that.