...Must Come to an End (2) - S6-E24
Revealing mistake: Immediately after Jen dies in the hospital room, Grams wakes up and notices something is wrong. She stands up and checks Jen's pulse, using Jen's left wrist. You can see a clear small marking/line of some kind on Jen's wrist, and Grams slides her fingers directly over this mark to check the pulse. Apparently the mark was a cue for Grams to know where exactly to place her fingers in the scene.
Answer: It's only explained once in the series. Jack says to Andie "I tried to start kindergarten without you", meaning Andie was smart enough to start school early.