Check Virginia law at the time. I think you will find that slave owners could emancipate any man slave not less than age 21 and any woman slave not less than age 18, and no slave older than age 45.

Children cost money to raise (feed, cloth, house, and supervise). The break-even age was about eight when slave children could do tasks equal to the value of their keep (source: Fogel and Engerman, Time on the Cross (1974), page 74).

A slave owner planning to emancipate his or her slaves would have two prime considerations:
(1) The freed slave would have to be mature enough and skilled enough to be self-supporting. State law required that emancipated alves should not become a burden on taxpayers to support
(2) In the case of a will or deed where slaves were to be emancipated at some time in the future, slave owners were unlikely to impose uncompensated expenses on their heirs. Slave children might be would be kept in slavery long enough for the slave owner who raised them to recoup the lifelong costs of raising them, which normally meant sometime into early adulthood.