Average Life Expectancy: Thirty Years...?
Ideas on what Thea's eventual fate might have been...
If my theory about Thea being the mother of the version of Kara in Nial's world is true, one must wonder what became of Thea between the second and third generations, as Thea is never seen in the quest of Nial's son.
As much as I hate to say it, my guess would be that she died at a relatively young age, before the third generation quest began. Perhaps she even died during childbirth. We know that Lune must have named Kara, because Kara's name is the same even if her mother is different. So, it seems very likely that Thea died before she could be consulted on what the baby's name should be, so Lune named her Kara.
It is also possible that Thea is alive during the third generation, but we simply do not see her. She could be somewhere on Alisa III, perhaps visiting someone she knows there. Could this person be Ryan? I would like to think so.
Or, Thea could simply be in an area of Dahlia that we are not shown in Phantasy Star III. But if that were the case, I would think that she would come to see Kara off before Kara joins Nial's son and goes off on her adventures. Then again...anything is possible.