In a synopsis released by the CW for the March 31st episode of DC's Legends of Tomorrow, we learn probably more than you want to know since we haven't seen the next episode… but needless to say that Sara Lance has reason to go see the League of Assassins in circa 1950s and meet up (for the first time) with Ra's al Ghul. Matt Nable will be reprising the role of Ra's from his time on Arrow.
That's the nice thing about having immortal characters in your universe. You can plug them in at any time and not have to worry about making them look younger or older.
DC's Legends of Tomorrow airs Thursdays at 8 PM on the CW.