So, this is my own Ais theory. I felt like I should make one after writing it out on Angelo Gabrini's Ais Theory Blog.
This is my theory.
Remember, Bell never knew about Ais' personal history; he only read the Dungeon Oratoria, which had a mention of Albert having a child. If Ais really was the child of Aria and Albert, then she must be over a 1000 years old, since the Black Dragon was forced into the skies during Ancient Times.
And since Ais was probably only 6 or 7 years old when her parents vanished, and over a 1000 years has passed since they did, Ais has be 1006 years old; but she isn't! She's actually 16 years old, so there would be an approximately 990 year gap between the two times. There are two solutions to solving this gap: 1. A Stasis-Type Magic cast by her parents 2. A Time-Travel Type Magic cast by her parents
Both of the solutions would solve for the humongous time gap in between; and both are equally plausible.
If the idea was a Stasis-Type Magic cast on Ais, then it would perfectly preserve her until the magic dispelled, in which she would wake up with no memories of the time gap which she had conveniently passed over. It would also account for the black tornados that sucked away her parents in the SO manga; dimensional transfer would cause her present situation to be whisked away, and since the dream cuts off after, it would mean she has no memories of what happened after she was sent into stasis.
Time-Travel Magic would also easily explain her present condition: however, instead of passing through the 900 plus year gap, Ais would jump over it and avoid all of it, however the black tornadoes can still be accounted for because of only her being sucked into the time vortex. It may be one of the side effects of Time-Travel Magic or Stasis Magic; those have never been seen or explained or even mentioned in all of the Danmachi universe.
Both theories are equally plausible.