I think it's high time she is told, but read this first:
Having a whole bunch of children, I can tell you that each child is special in its own ways – which includes that each one has their own set of problems. One is good at math, but horrifically bad at remembering even a dozen country's capitals. One is very well aware of what the people around feel, but did wet the bed well into school age. One is great at sports, but bad at paying attention. One is always very helpful, but also very impulsive...
So this one child of yours has this problem. I am sure her brother has other problems. In fact, if you talk to your daughter's peers' parents, you are very likely to hear that each of their children all have one or more problems. Only their problems are different from your daughter's. So your child isn't standing out for having this problem. She just has a different problem then everyone else – just like everyone else has.
And one more things: One of my friends suffered from a severe learning disability in his childhood. He's past 50 now, and it still shows. But while I would not ask him to review a manuscript, and while he's never had a well-paying job, he is a very wonderful human being, who is great to be with, loves to help other people, is exhaustingly thorough in everything he starts, never forgets anyone's birthday, and what he does as an occupation he is very good at. In short, he is a genuinely great addition to his social circles, even though he has, just like everybody else, deficiencies. He certainly is much nicer to be around than some very bright people I know which I sometimes loath having to spend time with.
If you look at your daughter's learning disability from these angles, you will find that she is not worse than other children, she just shines in a different light. (I am sure you have known this all along, but it is important to consciously think of it once in a while.)
It is this thoughts you should have in mind when you explain to your daughter what her special problem is. Make her aware of her weakness without making her feel like she isn't worth much. Explain to her that we all have our strengths and weaknesses. Point out what she, her family's members, and her friends are good at and what these people are bad at. And do not fail to point out that the latter, our weaknesses, is exactly what we all need to work on harder than the other people need to.
The latter is the reason I think you need to explain to her as soon as possible what her problems are: How else is she to know what to work more hard on? The fact that this does not make her an unworthy person is the reason I think you do not have to be shy to tell her.