Hi Beulah,
thank you for your answer. Going this route has been very helpful. I looked into the insane asylums and whatnot and found that in 1883, Dr. Charles Hoyt conducted his annual report of the condition of the state poor houses in New York. In his summary of the conditions at the Steuben County one, he states that there was a feeble-minded girl of 16 years who had been admitted in the facility less than a year prior who was the mother of a six month old baby boy. He stated that she was getting the paperwork done to be sent to the Newark Custodial Asylum very soon. I think this could be my Maggie but I can't be sure and cannot find any more convincing information.
Having looked back at the other census records, I just realized that her parents and older siblings did not seem to know how to read or write by 1880, at least not in English. All this, I hope, should lead me to a well informed conclusion about her life.
Mike