I would not tell at that age, primarily because you can't take it back and I can see a lot more potential downside than potential upside.
Also (as has already been mentioned) it's not a perfect measure of anything except how someone performed on one particular test on one particular day. This has value, but I don't think a child should form their view of themselves around it.
I do think it's important to let a child know if they are very intelligent. I imagine that typically this will be mostly confirming the child's own observations, but not always. I also think it's important, at an older age, to discuss the implications of a very high I.Q. vs. a more 'typical' gifted I.Q.
Another issue is that even if the child is a consistent scorer with no particlular weaknessses, a measure of their ability vs. "the world" might not be an accurate gauge of their ability vs. their immediate peers. Which could give a child a very highly inflated sense of their abilities relative to their friends and classmates. My I.Q. was measured once at 142, which would correspond to the 99+ percentile. But in my mediocre HS senior class of about 120, at least 2 other people had SAT scores as high or higher (old SAT).
Kids are going to start to see other test results and will form a picture over time. I know the PSAT and SAT aren't as correlated with intelligence as they used to be, but there's still a correlation and kids (meaning OTHER kids) will start getting the picture in HS, if not earlier. And there will probably be other standardized tests along the way. I think the parents' role is to make sure that picture is accurate and to help the child understand the implications of that picture. But I don't think parents should provide a difficult to interpret number prematurely. I wouldn't trust a 10 year-old not the throw the number out to his/her friends. And that's likely to cause confusion and resentment.
