William’s will appoints his wife and his son Thomas as joint executors, and also makes it explicit that Thomas was not of age: lands were left to him when he became of full age. So there is no difficulty about his being a minor. As Vance has said, there was no legal impediment to a minor being named as executor in a will, though they would not be allowed to act themselves as executor until they became of full age.
As an aside, the position now in English law is not very different. The current legal position is that, if a named executor is under 18, it is the adult named executors who handle the estate - with special arrangements made in some circumstances to protect the interests of the minor, and with the minor becoming able to act as an executor if the estate is still being administered when they reach 18.