Go to your local largest library, and have them order you a SoundEx code Microfilm for 1850, OH, for the names of WILSON.
On the film, you will see images of indecx like cards.On each is the name of a person with a name that 'sounds like' WILSON.If it is a whole house hold, the name of the head of the house hold is first, and then each person living in the house hold.It will give their ages and the state where they were born, and the county that they are living in at the time of the 1850 census.
By viewing these cards, you should be able to find your Mary.She should be listed as either 14 or 15, depending on when the census was taken.
It will cost you about $4 or $5 to rent the film for about a month, and it takes about 2 or 3 weeks for the film to come in.
This method has helped me on a number of occasions.