[@AimeChambers] We should totally post in the order of Muse A, B, C, D 

we don't have to... just ocd xDD

And yeah, Jem starts getting his shit together during paranoia time: Highschool years Sophomore year to Senior year
Then he had a coping period to deal with the loss of his parents but that didn't stop him from going to college. 
Once he was officially done with college, he was very sure of himself and all his sadness was turned into positive ambition. 

So I'd say Sophomore year/Junior year of highschool was probably when Jem found out about Eric's dark secret. However, it was at a good time where he wouldn't use violence to comfort his friend. He would be the support his friend needed him to be and tell him, he'll be there for him always.