Please do not file early. It does open the way for the state to start encroaching other requirements on homeschoolers.
As for Colorado...
I am well acquainted with the laws there. They changed the
law in 2007 but did not make changes to the cde website. So, if your child is 6 before or on August 1 of that year you must file your
notice of intent . If your child's birthday falls after that, you may file the following school year. You do not have to begin your school program until the age of 7 however.
If you click on the link it will show a sample notice of intent. Please do not include any other information other than what is required by law. They may say that you need to include a list of books or ask what curriculum you will be using or even send you one of the school district forms to fill out, but stick to what is written on the CDE website. They tell you only name, age, hours or instruction and address of the children.
You can test administer the test yourself. I used to get mine from
here. Or you can use a teacher for the assessment. There are several throughout the state that are really hs friendly.
Colorado is pretty lax in their hs laws. I would join a hs group if you have not already done so. What part of Colorado are you in? I lived in Aurora. If you are close to there I can recommend a great group.
You can also register with an umbrella school such as CHEC. If you do, you will no longer have to worry about sending in your notice of intent to the school district.