So there’s a lot of confusion surrounding the presumption that BYDTW Day was the day GLaDOS took over the facility, and how Chell managed to grow up if being in stasis prevents aging, and where her personnel file came from.
I have two completely different theories for this.
Theory 1; Minors can’t be tested without a parent’s signature. Every day in captivity Chell filled out an application to test and was subjected to interviews and examinations to determine what kinds of trails she should be put in, which is where her file comes from. At the end of the day, she’d be told they had to reject her and to please come back tomorrow with her parent’s permission, and be sent back to the relaxation center. Thus, GLaDOS couldn’t test her, but couldn’t kill her with neurotoxin for refusing to test either. When she turned 18, the system started rejecting her and asking her to come back when she was less tenacious.
Theory 2; BYDTW day was the day the Combine invaded. Aperture initiated a lockdown as one of their many apocalypse contingency plans. While trapped inside, the scientists had to develop a means of combating the Combine so that the system would let them out. There were no shortage of volunteers willing to test to speed things up, but they just never had the breakthrough they needed. After about 10 years without significant progress, someone finally decided that they needed some direction. They needed a leader. So they started tinkering with the Morality core that they abandoned after the BYCTW day fiasco. The next time they put it on GLaDOS, it seemed to work. They informed her of the situation and she quickly decided that if they really wanted to speed things up, they couldn’t rely on volunteers anymore. Everyone needed to test and leave the thinking and engineering to her. Then the itch, euphoria, and withdrawls came. GLaDOS realized she was in a pretty good position now. Nobody could leave until they developed some technology or strategy that would allow them to save the world, and they were relying on her to guide them to that discovery. She could test them all to her hearts content, forever, and make the tests as sadistic as she wanted, and anyone who told her ‘no’ would be putting themselves above the rest of the human race. People caught on to this quickly and tried to shut her off or escape, but it was too late for that after the first picosecond.