I suppose it depends on the severity of the condition, but I was wondering how normal it is to go through several years of school before a "problem" is noticed by parents and/or teachers or other professionals. Is this possible?
Autism is strange beast. It is extremely varied from person to person, and other psychological disorders (and several genetic ones) can look like Autism and not be it. Depression is a good example. Both have very similar symptoms, and at times it's a toss up between which one it is.
3 is around the age where they make an actual diagnosis (also) I believe, but if you have a good pediatrician they start looking for signs much earlier, 1.5 to 2 years old. You should have been asked questions, or given a list to fill out before the child's 2 year checkup. (now depending on the child's age, this may not have happened, as within the past few years, they revised the checklist to include children of a younger age.) You can ask your child's doctor for an Autism checklist they give to parent's to see if concern is warranted.
Children can technically move in and out of this diagnosis, it's just that most don't. Unfortunately, there is no genetic test etc. to say, "Yep, that's what it is." On the upside that means that some of those who go through counseling etc. can lead a somewhat normal as opposed to things like Down's Syndrome where those people will almost always need some sort of help.
It is definitely possible for this to get "missed". Teachers and doctors who are overworked or just don't care could miss early warning signs. The problem with Autism is that it covers a range of behaviors. Depending on the severity, a mild case might be labelled as quirky, or shy. 30 years ago a child who was considered an outsider, might be diagnosed with Autism now. Asperger's syndrome is an Autism spectrum disorder, and people with this are capable of functioning in society without difficulty.
If you have health insurance, what I would do is explain to the child's doctor your concerns and see what comes as that. Check with your insurance provider and see if you can have a psychologist do a formal evaluation. You can also check with the school to see if they have someone who can do that also. In the U.S. most places have developmental schools which are specifically geared towards diagnosing and helping children with disabilities. I suggest this advice more than all others: have two people evaluate the child. Autism can be very very difficult to diagnose, because it focuses on a range of behaviors. One person may say no, where the other may say yes. If you suspect something is wrong, and the first says no, go to the second one. I know this sounds like you are shopping for the answer you want, but in this case you should be absolutely sure. The sooner the better too. The earlier treatment starts, the better off the child will be.
Above all, you need to have professionals evaluate the child. Don't try and make a formal judgement yourself. They know what to look for, and they know what other problems look and what to look for.
Here is what you have to remember above all about Autism. The child is still your kid. Autism is nothing more than a name given to a series of behaviors. Don't let a formal diagnosis change how you feel. All it is saying is that the person needs some help in certain areas. Think of it like getting a Math tutor, but it's tutoring for social skills. Most children with Autism aren't slow, they aren't retarded, they just have difficulty in social situations. If you haven't heard of Temple Grandin, I would check here: http://www.templegrandin.com/templehome.html
Sorry but I have to disagree with Kevin based upon the evidence.
Autism is generally diagnosed around the second birthday and as another poster said, there is a wide variation and severity of problems, which make a child at one end of the autistic spectrum very, very, different from a child at the other end of the spectrum. The problems can not simply be explained in terms of "behaviours" like "needing a math tutor" - these are a set of problems caused by abnormal neuro-development. It seems that in individuals who have autism, parts of the brain wire themselves together differently. - It now seems also that there can be various causes for this, one of which is genetic, but also autism can be caused by brain injury.
Neither are the problems caused by this abnormal wiring pattern just "social," they are wide ranging. The fact is that the majority of children who have autism also have some degree of learning difficulty; - some also have epilepsy; - many experience sensory processing difficulties, which then have the effect of producing obsessive, compulsive, defensive and other behaviours.
As to your point about being able to go several years without is being noticed, - with some very mild cases, possible. There is a good book which describes many of the problems which are faced by autistic individuals, which you can find it here.
This is a very difficult question to answer, for the following reasons:
Finally, as Kevin already pointed out, many other things can be confused for autism. A number of speech/language disorders, emotional trauma, sensory integration disorders, sensory processing disorders, extremely high IQ, a number of social issues, and several behavior disorders (just off the top of my head) are commonly mistaken for autism.
As a very general guideline: I'd be very leery of someone offering a "definitive" autism diagnosis under age 6 or so, and I'd be equally skeptical of an autism determination for a child who showed no symptoms during early schooling and suddenly developed problems later on. While it can sometimes be difficult to discern whether a young one's problems are related to autism or not, unless it's a case of high-functioning autism or aspergers being initially masked by a very bright child's intelligence, it's hard not to know that something is up long before the child is done with primary school.