Definitely not my area of expertise, but this might help a little.

I read a biography of Harriet Tubman by the historian Catherine Clinton some years ago and was equal-parts surprised and horrified to learn that her owners first hired her out when she was only five or six. (Her date of birth was never formally recorded, so Tubman didn't know exactly how old she was, and historians continue to disagree on her most likely year of birth.) She was still a very small child herself, but was expected to care for the infant of a neighbor to her owners and was beaten whenever the baby cried. From that point on, she either worked full-time for the Brodess family (her owners) or was otherwise hired out to other plantations in the area. It was her owners who were paid for her work, so there was certainly an economic incentive to get a slave working as early as possible. If I remember correctly, Tubman was allowed to keep a very small portion of her wages for day-work as an adult, but not as a child, so of course the incentive to hire out a child slave was just as strong even if they didn't bring in as much money.

Tubman's brothers and sisters were first hired out at similar ages (again, near as we can tell). Greater age and maturity required more mentally and physically demanding work, but even small children were considered perfectly capable of plucking game birds, hauling firewood, checking traps, and carrying water for field workers. Tubman seems to have trained for a number of different tasks both inside and outside the house as a child/adolescent so she could hire out for a wide variety of work, which increased her economic value.

I'm unwilling to extrapolate beyond that, but this doesn't appear to have been an unusual practice on the Eastern shore of Maryland in the 1820s and 1830s, at least.