One of the most common questions on the AmblesideOnline forums and Facebook page is some variation of, "I am switching to AO with older children.... where do I start?"
Although there are always variables in everyone's life, and sometimes they lead to out-of-the-ordinary suggestions, for most people the question usually gets a standard answer. This is my usual answer.
Because Charlotte Mason wanted children's minds to be fed, I always recommend starting with your child's ability to understand, not their ability to read. The books in AO are well above modern "grade level" and are not expected to be read completely by the child, especially in the early years. Instead of going by reading level, I always recommend going by comprehension level.
Recognizing that the AO years were not designed as strict grade levels (although Year One does match up with grade one if a child starts it at age six) the usual suggestion is to pick a year or two lower then your child's "grade" and look at them for comprehension first, and then content.
Start with comprehension: One suggestion that I like is to pick a couple books from the years you are considering, and read a passage from the middle somewhere. Can your child narrate it? (If he's new at narrating, this might not be very good- just ask for three things he remembers.) If that went well, consider the other books- what are they about? What subjects are covered? Do they sound like they are too hard for your kids? If it comes down to a choice between a couple of years and you can't decide, consider the history chronology last. If your child just learned all about the middle ages, you might not want to do Year 2. But that's the last thing to consider because it's of less importance then placing your child where they are challenged, but not overwhelmed.
If you are new you need to watch this video. Watch it. There is a lot of information here that is easily missed, and a lot of explanations about the different parts of the site and the options you will see.
If you would like more information on the AmblesideOnline books and grade levels, here is a short post addressing that.
Some advice from experienced moms on placing in Ambleside. (I think these are from the old yahoo groups, you can find tons of similar advice on the forum.)
Here is the AmblesideOnline history chronology, which is really helpful as a broad picture view of AO.
The AO FAQ.
The GORY DETAILS from Afterthoughts (This is very helpful if you are getting started.)
And maybe most important, the Forum. This is where you can get free personalized advice on implementing the curriculum in your own family, from experienced AO users, including the Advisory/Auxiliary.