Yes, never too early to start. The question is the level of intensity of prep.
Example - my D first took the SAT in 7th grade, as part of a talent search. This got her familiar with the test format, types of questions and, with the QAS service, the ability to see correct answers where she got questions wrong. She took it again late In 8th.
I think this early exposure allows the concepts, process, etc., to sink in over time. Already being a Sophomore, there aren’t and “free, doesn’t count’ testing opportunities, so I would take one of the practice tests now to get that familiarity. Then, use the diagnostics from Kahn and do light practice on an ongoing basis.
With that, I’d ramp up prep about 6 months before the planned test date, and then really focus over the final 6-8 weeks. I like to have the initial test be in the fall of Jr year, giving the free time in the summer to study.