I haven't really been able to find specifics from the design journals, although almost all of them seem to talk about plot lines within the context of a single character, not a soul.
It's possible, and probably necessary due to the quantity of players, for storylines to stretch out over multiple lives. I feel like each new character would probably have to meet a certain threshild, say an age or skill, before they could continue their story, though.
For example, I doubt a freshly sparked 13 year old teen would immediately be able to utilize the skills necessary for completing a certain task your previous character had set forth to complete, like forging or combat.