Squad Stories
Squad stories can be unlock if you have one member reaching the rank of corporal, so check there rank. Also you can have Curtis and Laurent at chapter one, and Eileen can be unlocked as soon as Curtis is down and rescued. So it is safe to say that like old times can be get as soon as you unlocked skirmish. (Spending fourty CP on a unit on the first skirmish map is the fastest way to go about it to get a unit to Corporal)
List of Squad Stories:
A Chivalrous Heart - Earliest Available: Final Chapter
Christel, Minerva
A Prayer for the Broken - Earliest Available: Chapter 3
Godwin, Nico, Rosetta
All the Single Ladies - Earliest Available: Chapter 6
Brittany, Millennia, Rita
Girl in the Iron Mask - Earliest Available: Chapter 4
Ferrier, Jean, Stanley
Honor, Pride and Regret - Earliest Available: Chapter Unknown
Fleuret, Mabel, Rebecca
Like Old Times - Earliest Available: Chapter 3
Curtis, Eileen (Special), Laurent
Reluctant Solitude - Earliest Available: Chapter Unknown
Louffe, Stahlschrott
The Price of Skill - Earliest Available: Chapter 8
Aladdin, Leonhardt, Neige
Treading New Ground - Earliest Available: Chapter 4
Aulard, Dan