@AshNanoDesu No, Souta's not 10 years younger. Didn't we just discussed those additional character introduction pages the other day? (which we will upload soon by the way)
They reveal that Souta is 5th year in elementary and Nei is high school 1st year. Which means a 4-5 years age gap, with Souta being like 11-12, and Nei like 16-17 or so. (though their age wasn't stated, but both school years, appearance and everything supports that theory, I think)