Owing to some confusion as to when events happen, I drew up
this timeline. Hope it helps a bit.

Timeline:
Time          Age               Events
-27                                 Ranma Inverse and Zelgadis appear
                                       in the past.

-23                                  Luna is born

-17                                  Lina is born

-9          Lina is 8              Lina, Akane, Gourry and Amelia
              Luna is 14          show up. They plus Zelgadis
                                        and Ranma use the Gate at Mount
                                        Phalos to travel to Japan and
                                        use the Mirror of Changes there
                                        before Ranma and Akane do.*

-3          Lina is 14            Luna finds the Mount Phalos
              Luna is 20          Gate on one of her expeditions.
                                         Lina takes the opportunity to
                                         leave Zefielia.

-2          Lina is 15              Most of the Movies occur.

-1          Lina is 16              Most of the TV series occur.

0            Lina is 17              Ranma and Akane appear. Ranma
              Luna is 23             and Zelgadis are lost in the
                                           past. Lina, Akane, Gourry and

                                           Amelia use the Gate at
Zefielia
                                           Castle into the past. Lina,
                                           Gourry, Amelia, Zelgadis,
Ranma
                                           Inverse return from Japan.

* this seems like a paradox but it isn't, because it's an
adjustment of the original anomaly and it occurs outside
the Slayers Universe.