My standard recommendation is that a recruit should register with the NCAA Eligibility Center during or shortly after their junior year of HS. There are a couple of reasons for this.
1. If the recruit is a strong academic student and has been taking a lot of college prep courses (which are basically what the NCAA defines as a "core course"), it's possible to receive Early Academic Certification before they even start their senior year.
2. The Eligibility Center will not only send out task lists to the recruit, but will send out NCAA educational information as well. Some examples are information about NCAA-banned drugs and rules regarding gambling. I expect that will also include information regarding the proposed rules that will allow athletes to be compensated for their Name, Image, and Likeness when it is approved. That legislation will be voted on this month and will take effect on August 1 of this year.
Also, even if a recruit is planning to start off at the junior college level, I still recommend that they register with the Eligibility Center when they're in HS. Even though their file won't be pulled and reviewed until they are being recruited by an NCAA program (as I shared earlier in this thread back in 2018), they will still be sent the educational info that I mentioned above.
Another reason I believe it's important to register while in HS even if a recruit is planning to start college at the JUCO level, is because a recruit needs to know whether or not they are a Qualifier based on their HS academic record. Their status as a Qualifier, Partial Qualifier, or Non-Qualifier will impact what they must achieve academically while attending the JUCO in order to be eligible when they move on to an NCAA program.