Alright, if the lifespan thing felt confusing, here is the easy version with a quick breakdown.
* Mundane humans, meaning people with no magical blood, just live normal human lives. Their soul is completely soft.
* Druids are born from one Firbolg parent and one human parent. Their soul is half hard and half soft. They live longer than normal humans because of that hard portion, but their lifespan is still limited. This is the most common type of magical human and basically the ancestor of most wizards.
* Firbolg-Born come from two magical creature parents.
- If neither parent has any kind of longevity trait, then they still have a fully hard soul and live around 700 to 900 years on average.
- If one of the parents has a longevity or immortality trait, then the lifespan increases to about 1,000 to 1,500 years.
- If both parents carry longevity or immortality traits, then the lifespan goes beyond 1,500 years and can even become indefinite.
- For example, Salvazsahar and Myrddin come from a phoenix and a basilisk. Both of those creatures are effectively immortal, so their lifespan reflects that.
Now for their descendants. Olde Ones, True Pureblood or Firbolg-Blood, Mixed-Born Purebloods basically in the same group. They still have hard souls, so they live longer than normal humans, but they do not quite reach the full lifespan of a Firbolg-Born ancestor.
The closer someone is to that ancestor, the longer they can live. As the bloodline gets more diluted and the soul softens, the lifespan drops. Modern purebloods, whose souls have softened a lot, usually end up with normal human lifespans.
Another thing to remember is that every Olde Ones family has one trait that never disappears from the line. It always shows up in some form. Like Keith's black hair with white or silver streaks, the LeFay family's emerald green eyes, or the Potters' messy hair. That trait only appears in people who actually inherit that family's magic.
That is why Lily has green eyes. She inherited the LeFay magic. Petunia did not, so she doesn't have it.
So in general, Olde Ones always have a visible trait that sticks around. It is usually the easiest way to recognize them, though it is not absolute. Other families can have strong traits too, so the more reliable way is still to look at their ancestry.
In the end, lifespan comes down to two things. First is ancestry, what kind of Firbolg ancestor they come from and how close they are to that bloodline. The main branch can match the ancestor's lifespan, while distant branches live shorter lives. As that happens, the physical trait also becomes less noticeable over time.
So in the end, everything ties back to bloodline and the density of soul