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