*In my fanfic, the timeline is really different than how it's portrayed because I started writing this before Season 7 was finished and I don't read any of the twitter sources on the wiki and just go off my own imagination. I'm putting a lot of emphasis on this.*
So, Lazuli was born a mixed hybrid Elemental Master, which means she comes from more than one Elemental lineage and possesses more than one Elemental power. Being the oldest, she takes the mantle of Blue Ninja after Jay retires when she's 21. Her younger sister Sapphire was born a few years after Lazuli was, and is a dominant hybrid like Nya, only with Lightning instead of Water. Sapphire became the new OFFICIAL Samurai X after Nya remade the suit after the original got stolen. Cobalt and Azure were born years after Sapphire was, and Neil about 5 years or so after them. Cobalt, peculiarly, has WIND and Water powers, while Azure has Lightning powers. Neil doesn't unlock his basic powers until he's older than Zane was when he became a Ninja (35 in my timeline).
Lazuli marries Logan Garmadon, the Second Green Ninja, and they retire when their oldest, Erica, is 13 and her sister Lea is 11. Lea becomes the next blue Ninja. I haven't figured it all out for the other 4 Jaya Kids, but I will eventually.