There were ages that were far more suitable when it came to accepting certain changes to allow them to adapt to new things, like a sudden change to their worldview.
Perhaps adults would be more mature when introduced to those things, so they would be more likely to avoid less harmful behaviour, but it was more likely that those within the age range Ash had set would be more likely to meet all of the standards that were required of them.
Even if there were people within a certain age range who were suitable, there would still be many with different world views and circumstances that would make them far more likely to accept the changes to their lives, and that would not leave them scared or confused about it.
That type of situation made it even more possible for Ash to narrow down her search further.
If they were able to adapt quickly, then they could begin going solo like Ash did when she first started, but they would not have all of the same restrictions.
