I think you are sounding more judgemental the more you write. She is young? So that proves...what? I was 21 when I had my son, 23 when I had my first daughter and 26 when I had my second daughter. I can guarantee you I never felt any urge to 'prove' anything to anyone. I have one child with special needs and learning disabilities, one who is highly gifted and one who seems about average. Age of the parent has nothing to do with it. You do not know for sure that this mother is not telling the truth. Maybe the school is planning to skip her daughter based on her birthday (maybe she just missed the cutoff), maybe she is very educationally advanced, maybe a dozen different reasons. Honestly this post just seems like it was written to give you a chance to prove something, not her.
I posted the question because I genuinely want to know how other people handle these issues, not because I need to prove something to this mom. There are things about which I'm judgmental, to be sure, but the age of parents isn't one of them. It's just an observation from experience with really young moms.