Wayne is referring to a Charles City County court record dated August 1694 in which Mary and John Turberville, identified as orphans of John Turberville, choose their guardians. (Mary chose William Gardner, John chose Robert Williams.)
This record means that the children were 16 or more (thus had the right to chose their own guardians), but not yet 21.
There could have been additional children who were over 21, thus didn't need guardians - or children under 16 who didn't have the right to chose their own.
However, I'd note that Turberville didn't marry Susannah Daux until after 1675 (when she was still married to Rawlins), and the court record above is only 19 years later, so there probably weren't any children as old as 21.
A Richard Turberville patented land in Charles City County in 1693.He couldn't have been a son of John Turberville and Susannah Daux, but it is possible, I guess, that Turberville had an earlier wife by whom he had older children.Not likely, as John was apparently born in 1649, but possible.
There was also a Robert Turberville claimed as a headright by William Randolph in 1691.
It's intriguing to speculate that John, Richard, and Robert were related in some way, but not likely as father and sons.There's no record of a later Richard in the court records, but he could have been a young son under 16 in 1694.