Records of Dettingen Parish, Prince William County, Virginia: Vestry Book, 1745-1785, Minutes of Meetings of the Overseers of the Poor, 1788-1802, Indentures, 1749-1782. Dumfries, Va.: Historic Dumfries Virginia, Inc., 1976.
http://wvancestry.com/ReferenceMaterial/Files/Records_of_Dettingen_Parish_Prince_William_County_Virginia.pdfIndentures start at image 111.
From reading these indentures, it appears that white children were bound until age 18 or 21, and mulatto children were bound until age 31.
Examples:
Image 114/page 106, shows twin mulatto boys bound until 31 years old. The boys were shown having surname Fullam, which was the mother's surname.
Image 120/page 112, bottom, shows a boy James, no surname, bound until 31 years old, and the mother was named as a mulatto servant, with surname Lucas.
If the Martha Hudman you're researching was not recorded as a mulatto or black, it could be possible that she was white while her son's father was a black or mulatto man.
However, again on image 120, the entry for Thomas Grigory, a mulatto bastard child, specifically states his mother was a white woman. a servant.
These records are church records, while the record you have for Martha Hudman was the court record. These two official agencies might have recorded these details differently. Martha Hudman doesn't appear in the church record. Francis Jackson does appear on image 119/page 111, as a witness.
