I think the John in 1810 and 1820 is John Mize Jr. John Sr. and Keziah sold their land at the end of 1801 and were off the tax lists in Mecklenburg in 1802. I think they moved back to Brunswick County where they had been before moving to Mecklenburg. A Lunenburg deed in 1790 shows John Mize and wife Keziah sold their Little (Aaron's) Creek land to Moses Dobyns, and John was called "of Brunswick County" at that time. The John Mize on the census in 1830 appears to be a possible son of John Jr. I think he married Frances "Franky" Hudson.
The John Mize who married Lemenda Lambert and remained in Lunenburg on his family land until the mid-1830s was the son of Stephen Mize. This John was born in the 1750s, while the John (son of James) who married Keziah was born in the 1740s. He and Keziah "Cissey" sold land together in 1770. He was also listed on a 1769 Lunenburg tax list by himself, so he must have been at least 21 at that time I would assume.