The first Portal starts at least a decade after the neurotoxin incedent, which happened on 'Bring Your Daughter to Work Day.'
In Portal 2, in the level where you see the abandoned science fair experiments the scientists' daughters made, there is one with a giant, overgrown potato.
Zooming in on a note on the right side of that potato's poster board will show that it was Chell's science fair experiment for Bring Your Daughter to Work Day.
So, GLaDOS killed all the scientists on Bring Your Daughter to Work Day and, assuming that since the room the experiments is in is closed off in glass it blocks any neurotoxin, she kept all the children in cryo-sleep as test subjects and made files for all of them.
So, Chell has been in the facility since she was probably somewhere around age 7 - 14.
....really makes you think what her mental capabilities really are.