The Age Differential
Most people with Aspergers seem to be "born with a knowlege of their difference", though in truth this awareness develops over time, mostly during their primary school years.
The age at which the label of aspergers is used to describe these differences seems to have significant impact upon the way in which they are received by the aspie. Of course, there are other factors too, particularly the way in which others around the aspie take the news.
The Very Young
As you would expect, telling an "under 8" year old child that they have aspergers is pretty similar to telling them that they are wearing a blue cardigan or that they were born in Australia. Their reaction is pretty minimal.
Of course, the fact that you have now disclosed this "secret" will cause them to pay a little more attention whenever the topic is discussed in the household. As a result, parents who have "disclosed" need to be responsible for maintaing a positive view of the condition. At age 8, it's the way that others describe the label rather than the label itself that matters.
The Teenage Years
We all know that teenagers are generally rebellious and angsty, they don't take criticism well and they struggle with self-image. If you attempt to diagnose a teenager with aspergers, they will either, "not want to know" or they'll take it onboard in a negative manner. The teenage years are probably the worst time of all to press a diagnosis.
When teenagers decide not to accept the diagnosis, they'll rebel not only against the label but also against any support you manage to obtain for them. They'll reject the idea of "remedial classes" or "special teachers" because they don't want to be seen by their peers as "different". There's not a lot you can do about this and they can't be reasoned with.
Worse still, is when teenagers take the label in a negative way. They can sometimes decide that the label defines their life and limitations. As a consequence, many diagnosed teenagers simply stop striving for academic or social goals.
Much as I don't favour withholding a diagnosis from a child, I'd be more inclined to keep things quiet if the child was a teenager. It's still worthwhile organising special classes but it's not a good idea to the let teenager in on the reason why these classes are being arranged. If you don't tell, they'll draw their own conclusions.
Like the teenage years, early adulthood can be a dicey time to reveal a label. Young adults are struggling with a different set of social issues such as dating, tertiary education and starting work. Rebelliousness isn't as problematic at this age but depression is. Handing out a label at this point is almost like handing out an excuse to stop trying.
Many young adult aspies seem to like putting "road blocks" in front of themselves. They'll talk about a problem, such as not being able to find a job or a partner. Then, when given advice, they'll find a reason why they can't use it. For example, they'll say that they can't get a job because they haven't got a suit. You'll offer to buy one, but they'll find some other excuse like "I don't look good in a suit". If you press against this logic, they'll resort to less and less sensible/logical arguements. In reality, they're depressed.
The label of aspergers should really be presented when the person affected is feeling positive. Aside from study assistance, it's generally uncommon for financial assistance to be given to "early adult" aspies. In that sense, it doesn't really help anyone to reveal the label.
Mid Adulthood and Later
By about mid-adulthood, people are settled into their ways and nothing is likely to have a great deal of negative impact on their self-confidence. They've either been successful (or unsuccessful) in life, love and work already.
It's quite common for adults to discover that they have aspergers because one of their children is labelled with the condition. That's how it happened with me. The initial reaction isn't one of shock, though a little bit of denial is common. The main reaction however seems to be relief. The label finally explains things that have occurred throughout our lives.
Adults in this siutation often say that they wished that they'd known about it earlier. I know that I did. Unfortunately, the truth is that we probably wouldn't have accepted it had we known earlier. Worse still, knowledge of what we are may have changed "who" we are. It may have resulted in depression or other negative emotions.
I guess the main message of this post is that the best times to reveal a label are either early childhood or mid adulthood. Anything in between increases the chance of depression or non-acceptance. Since most adults would have preferred to know about the condition earlier in life, it follows that the best time to tell is probably around ages 5 to 10. If the condition is given both a positive spin, and a healthy dose of "indifference" (ie: telling the kids that it doesn't matter - and that it doesn't change who they are or what they can be), then the overall reactions and long term effects are positive.