Look up the law in your state or call the school district office to find out what the policy is. Here in Idaho, they could not be tested to start early if their birthday was after December 31. My son's birthday is February 3rd. So I had a different problem. He missed the testing deadline by a month, and the regular deadline by 5 months!
But I knew he was ready, even though he was only 4 1/2. He could read lots of words on his own, and, well, he's just smart, and mature. I felt helpless because the law wouldn't let me do anything about it. At least that made my decision simple, though!
Fast forward 1 year. He started kindergarten at age 5 1/2, and was reading 2nd grade chapter books. He still had fun in kindergarten, because to him it was like a daily playdate with craft time and snacks! But he didn't actually learn much.
Fast forward 1 more year, to Fall 2010. He was so far ahead of his first grade class! He started to get into a little bit of behavioral trouble because he was bored. I was on high alert for this sort of thing, so I took action quickly. I jumped through all sorts of hoops and got him tested, and he skipped first grade. Now he's where I wanted him to be all along! Ugh.
I have a late July birthday, and I started kinder when I had just turned 5. I never knew that I was the youngest in class. I was a smart kid, and knew that my birthday was last, but mostly I was just bummed because I never got to have a class party. That's all I cared about. I graduated from high school when I was still 17, and that's the first time I realized that, whoa, most people are 18 when they do that.
If you feel like she's ready, I would have her tested. What harm could it do? If she doesn't pass the testing, then well, she just got to have a fun outing playing letters and numbers with a nice lady. Usually, they'll tell you that you don't have to make your decision until after the testing, too. So deciding to have her tested doesn't mean you HAVE to start her this year.
Trust your Mommy instincts. You know what your daughter is ready for much better than any school district who just had to draw an arbitrary cut off line.