Dr. Stone and Jeff have a discussion about age and school at the beginning of the episode. During it we learn he is now 17 (Dr. Stone says "I've been waiting 17 years for you to get big enough . . "). We also learn he will be going to college next year, which implies he's an high-school senior.
Yet, just 8 episodes ago (Season 5, Episode 22, "Big Sixteen") he was just turning 16. So, in 8 episodes he's gone from his 16th birthday to now being somewhere in his 17th year.
Jeff Stone: (Mrs. Stone and Jeff talk while he is cleaning outdoor barbeque pit, without anyone asking him to do it.) Mom. Do you think if maybe I slip this whole thing up and wash the windows, a little money might exchange hands.
Mrs. Stone: A day's work is worth a day's pay.
Jeff Stone: OK, Thanks, Mom.