Just think carefully about how you will monitor her use of it. Personally, I agree with others that a desktop, with a nice, visible screen, in a public part of the house, is much more sensible. Here's why ...
DD1's friend has a laptop, which she uses in her bedroom. It is her dad's old one, which she was given for school work. She's had it for a couple of years and is now eight years old. So she probably was 6 when she got it.
She spends hours on it. She is highly at finding internet-based games, and has been pestering DD1 to register on one particular games, so they can play together over the web. This would require providing an email address and other personal details. The game also has chatrooms. She has also managed to view some pretty unsavoury images by googling for words that a child would find highly amusing (think toilet humour).
From what I've seen, I am very concerned that this child is getting into territory that she completely lacks the emotional maturity to deal with. She's really a very innocent character, but with highly-developed googling skills. Not a good combination.
Which is why we won't be going down the laptop route any time soon.