In the court documents on the guardianship of Capt. William Twitty's children, it shows that William, Susannah and Allen Twitty were allowed to choose their guardian (and chose William Graham), and that Graham was named guardian of Russel, Bellariah, Charlotte and Arabella. This suggests, as indicated, that Allen was older than Russel.
Russel's age estimate seems to come from his Revolutionary War pension application, where he indicated he was about 62 years old. As far as I know, no other record of his birth year has been found. However, he didn't marry until 1790 -- when he married Polly Mills, d/o Col. Ambrose Mills and Anna Brown.
We recently learned that, according to a book on Revolutionary War veterans burial sites, Russel was buried in the County Cemetery in Franklin Co., Mo. I asked the Meremec Valley Genealogical Society about the cemetery but they did not know where it was.
If anyone does know where the Franklin Co. Cemetery is, I'd love to know. Thanks. Sandy