He's 17 in JP DM 4 and DanMemo. Yen Press is a terrible source

I believe the story of Ais being 1000 years old is misleading.  I think she has memories of that time, or implanted memories from someone else.

It is more likely that the current Ais was born FROM the dungeon itself for a purpose, maybe similar in the way the Xenos came to exist.  However, in her case, since she has spirit blood, it is likely the dungeon was waiting for the right time to spawn her...as part spirit.

To be a little more clear, I think the Xenos themselves are part human, part monster, given lingering souls from those killed in the dungeon (a fusion of human and monster souls).

Since Ais was found in the dungeon, alone, it is possible she was spawned at a certain time, and possibly even in front of at least one of the loki familia captains...and it has yet to be revealed to others, including Ais herself, of how she was ACTUALLY found (a top secret that only certain persons are aware of).

The truth may be that Ais either died long ago in the dungeon, or she was created from someone else that died long ago, at the age of 7, or she may have been absorbed by the dungeon.  the honest truth is no one truly knows everything about how the dungeon works, not even the gods, and since she was found in the dungeon as a 7 year old girl, with no one having seen her before wandering around at a younger age, the best guess would be the dungeon itself being the answer to her only being 7 yet having memories from long ago.

so far, there has been no evidence that time traveling/portals would exist in the story or be possible.  cryostasis/cryonics is not that likely either as someone would have noticed her sooner.

I am sure in the near future it will be time to reveal her origiins and method of preservation in the main story (Bell has just started to see a possible connection between her father's last name and Ais last name at the end of LN 15).  I also have a feeling that Hestia will eventually have to tell Loki about Bell's secret, reveal how he feels about Ais, and the fact Ais has helped him train to become better.  I have a feeling it will be after Loki finds out from another god who Bell's grandfather is, and it is likely that as a result of learning those secrets Loki will share who Ais parents really are...and maybe a hidden item in her status that no one knows about.

That makes no sense:

1. Spirits don't come from the dungeon. I don't really have anything to say about the Xenos but they'd be more like Humans born as monsters rather than a Human and monster fusion

2. Albert didn't find Ais in the dungeon

3. The author can invent stuff whenever

4. Why would Loki do that? Also, Zeus confirms that Albert had a child

Really? I didn't know that, but it still doesn't change that there may be a connection between Aiz showing up and Welf getting his skill.

As far as we know memories can't be implanted, but I'm not opposed to the idea of Aiz being somehow coming from the dungeon itself, either because she was held there against her will or the Dungeon somehow decided to protect her and letting her go once it decided enough time has passed or someone who would take care of her was nearby.

The way I interpreted Ouranos when he explained to Hestia was that the monster's soul behaved differently than human souls and that somehow the ultimate administrator for monster soul reincarnation resided with the Dungeon itself. So I don't think a fusion of souls is happening and in either case I doubt that is related to Aiz's story.

This may be related to the secret that Gods seem to know about the Dungeon that they don't want to tell the children.

Zeus is only the adopted grandfather, it isn't possible for Gods to have children with humans. So I doubt that would affect Loki much. I agree that she will find out about the conditions behind Bell's skill and try to interfere, but she's more likely to stop Bell from finding out the truth rather than tell him and Hestia herself. Though I don't think anyone can really predict what the trickster goddess will do. The hidden item in Aiz's status may be an interesting possibility.

Uh no, there isn't. At this point you're basically reaching

Ais isn't 1000 years old, she's 16. Just because you were born that long ago doesn't necessarily mean you'll end up being that old

This makes no sense since the one who found her in the scene you're referring to was Albert. The Loki Familia members found Ais in the winter, which wouldn't really apply if you're in the dungeon

Ais wasn't found in the dungeon, and according to Zeus Albert's child went missing during the battle on the surface. We don't know of Ais' past because the author hasn't gone there yet, not because she was created as a 7 year old. Just because something isn't mentioned doesn't mean it didn't happen

All theories are a reach, but that doesn't mean they're impossible :D

Sure, but it matters whether or not it has a substantial basis, or whether you can answer the "why" and "how" properly

True, but I don't think we know enough about spirits to really say anything about "why" or "how" or substantial basis.