I have been reading some advise here and maybe you are able to help me too.
I have a cousin who came here to give birth. She did not overstay or work here in America and no intetion to became illegal allien.
Obviously she is not an american citizen and wanted to be petition by the baby she was about to born in November. My question was I believe her usc baby can only petitioned her at the age of 21. But can she file her paper now? Can she file the 485I in December since her baby was already born. But they say if you file 485I you have to stay in America but since she can stay on for a few months she has to leave the country so she won't be in that 3 or 10 years bar. How can she change her status if her daughter is only 1 or 2 months old. Is it posssible to file 485i even you are not in America. I would appreciate any advise. Thanks.

A USC cannot sponsor a parent until the child is an adult. Your cousin could only file an I-485 if she had an approved I-130/I-140; her daughter's USC status does not entitle her to stay in the US.

Unless her baby is 21 years of age, she can not be sponsored by the child. She is not entitled to file I-485 Adjustment of Status petition unless she has a pending I-130 and Visa is immediately available. She must wait till baby is 21 years of age.
Good luck.