it's the recommended level for starting dawngaurd, and is when guards talk about them giving the side quest to start the dawnguard questline.

However if you know where you're going it can be started at any time you want, even as soon as you get out of Helgen. But it'd be a bit weird to do that since you'd have a lack of weapons armor and experience.

And of course at a lower level than 10 you would have almost no chance of defending your towns against vampire attacks, but some would never bother thinking that far ahead and just go rush off and start the Dawnguard questline.