Your family were former slaves, so they won't be in the Census before 1870. In the book "Somebody Knows My Name, Marriages of Freed People of North Carolina", which are records of the former slaves as reported to each County after the Civil War, here is their marriage in Person Co.:
WASHINGTON MINNIS - PRICILLA BAIRD - Married in 1856
What you need to do is to research for possible slaveowners first by trying to locate any MINNIS slaveowners in records prior to 1870, and then by looking at their wills & estate records for the names of any of their slaves. While not all former slaves took the surnames of their slaveowners, it is a place to start, and the MINNIS surname was not a very common name. In looking at the US Census 1790-1940 for the surname of MINNIS, and then narrowing it down to "white" starting in the 1870-1860 census, you find that the majority of the names were in Orange and Chatham Counties. It is more than likely that your MINNIS may have come from one of the Orange Co. families, and that's where I would start looking at the Slave Schedules in 1860 and 1850, to see which ones of those families owned slaves. Once you've done that, then start looking for their wills & estate records and reading through them for a slave named WASHINGTON or GEORGE, and if you're lucky enough to find an inventory in the estate records, they sometimes include the ages of the slaves.
FamilySearch has some of the NC Estate Records and Probate Records online for various Counties, although not all of them are indexed, so one must do their own searching through the books for the surnames of interest. They have Person Co. Estates indexed, but I see no MINNIS, they don't have Orange Co. indexed. Both Person & Orange Counties are among the Probate Records, but none are indexed. The books that aren't indexed can be used by first looking for the time period of interest, and then seeing if the first few pages of the book has an index of the names included, then going to the pages indicated.
It takes a while, but I have been very successful at finding slave names and would advise that if you are truly interested in finding your family during slavery, it is well-worth your time to go through these various steps to locate them.
The link to the FamilySearch North Carolina Collections, which are free, but do require that you register to create a login to view images is this:https://www.familysearch.org/search/collection/list#page=1&a...