It certainly won't affect college entry 4-5 years from now. Colleges only look at SAT scores, and College Board discards any SAT scores before 9th grade unless you specifically request they be kept.
My DD14 took the SAT in 7th - there were many more date options and it was the only one accepted by SET. She took it in early October, so had only finished Geometry and a month into Algebra 2.
I had her take an actual sample test from College Board - full duration, break timing, etc. - and I gave her a 30 minute primer on Trig (sohcahtoa definitions and a few sample problems).
With that minimal preparation, she was fine with the test, qualified for the Grand Ceremony, etc. My assumption from being on this forum is that your kid is in the upper few percent - I'd recommend going right to the SAT.
