Keep in mind the age on the 1870 census might be incorrect. Whoever answered the door gave the census taker the names, ages and places of birth. If no one was home the census taker often asked neighbors. It is not far fetched to think that whoever answered the door may not have known the exact age of the domestic servant, Mary Greenwood. Many articles have been written about the reliability of information on the censuses.
Don't get locked into an absolute like she was 19 in 1876 so that means she was born in 1857. Adding/subtracting five years gives you a range of 1852 to 1862 to examine and a better chance of finding your ancestor.