You did not miss anything - I don’t know whom the writing team was doing the math, but it’s all jacked up. At MOST they should all be 18. And that’s even stretching it because seasons 1-3 only took place over a couple months. And Sarah was still 16 at the end of season 2. But she says in THIS season that she’s 19 and in John B’s speech in the finale, he implies that JJ is 20, which makes even less sense.