We don't stop playing because we grow old; we grow old because we stop playing.
In combat, leaders die with their last army in battle.
If you look into the clarifications file you see:
The way leaders are eliminated allows two opposing leaders to remain in the same area as the last units there. Whenever such a situation happens at the end of the combat phase, the active nation must retreat its leader.
... so, it's possible to have a leader alone in one area.