The best time to begin contact with your ALO would be when you begin your pre-candidate questionaire. This is also the same thing as your application to Summer Seminar, if you are interested in doing that. The timeframe for this is usually January of your Junior year in high school. If you contact the ALO before that, you run the risk of being ignored, or at least put on the back burner, since each ALO is busy helping out the candidates for the current year. You would also not want to wait until later in your Senior year, since the ALO can be very valuable in answering any questions that you have during the application process.
One thing to always keep in mind is that the ALO's are volunteers. They are current or former officers who are familiar with the process, and are giving up their free time to help you out. There may be times when you feel that they are not getting back to you as quickly as you would like, but remember that they are also helping many other candidates as well as working and managing their own lives at the same time.
Good luck with your goal of the Air Force Academy.
Everything Stealth 81 said about ALO's is true. However, there are those rare exceptions and we got one of them.
He only had a few kids up in our area. Both of them, unbeknownst to us, called USAFA and asked for another ALO (who just happened to be a good friend of ours). We didn't know we could switch IF there was a problem. We had several with him, or I should say our son did. Apparently the other two kids received great advice and follow-through. I could tell you stories to curl your hair.
Short story, 99% of them are fantastic. Take care that if you get that 1%, and they are not helping you at all, call USAFA and get reassigned.
I agree with what's already been said, but will add this. My son is a sophomore, but last year as a freshman spoke with his ALO at an "academy day" presentation. The ALO, the BGO (for Navy) and the liaison officer for West Point all encouraged him and suggested his "starting early" was a positive point. I think the ALO will either encourage you or say "check back when you're a junior" but I don't see how it could hurt you to indicate your interest now. Regardless, you'll know for sure about your ALO and what s/he thinks!
Absolutely agree. One cannot start too early. Academy Days are great places to meet ALOs/BGOs/MLOs. Check in with them. Know who they are. A "check back when you are a junior" remark, however, must be accompanied by a "put yourself in the position to take the highest level math and science courses your school offers" comment because the end of junior year is too late to cram in AP Chem, Physics, Pre-Calculus, Calculus, and a few others.
A jaded observation is that a lot of my pre-junior year contacts seem to be Academy Day attendees who have heard the above remark who are looking for my support in their rationalizations, excuses, and reasons for not taking calculus.