Yes, I think that your David J. Knowles (b. ~1784) is the brother of my ancestor Stephen Knowles (b. ~1800).It's interesting that Stephen names his younger son David J. Knowles in 1831. You may check with the Sampson county historical society.Jerome Tew has published a book regarding the service of Revolutionary War soldiers from Sampson County.He sent me an email telling me that John Knowles was awarded a disability pension after the War for wounds suffered at the battle of Rockfish Bridge (about 1 1/2 miles from my home) as a member of the Duplin militia.Also in the Duplin militia was a Robert Knowles who is listed as "born in Scotland"; a Robert Knowles is listed in the will of Robart Knowles who owned land in Bladen county NC in 1737 and subsequently in Duplin; the elder Robart's will was probated in 1763; another Robert (I'm assuming this is the elder Robart's son) made a will that was probated in Duplin county in 1792.I am assuming that John (will 1804) and Robert (will 1792) are brothers and sons of Robart (will 1763).There is a Robert Knowles shown in the Sampson census of 1790 (but of course not later since he died about 1792); John and James Knowles (both listed as being older than 55) are also listed in the 1790 census and in the 1800 census.But there is very good data on John Knowles's Revolutionary service since his disability pension is from the Federal government - you only need to show your direct descent.
What is the data showing John Knowles as being born in 1721?The only indication that I have of his age would be that he was older than 55 in the 1790 and 1800 censuses.That would make him about 82 years old at the time of his death about 1803.
It is very interesting to hear of David Knowles moving to Georgia and Florida.I don't know of the descendants of any of the other three brothers (from John's will - 1804) although some untraced Knowles men begin to show up in documents from the 18-teens in Sampson and Duplin counties.