Basically I agree with your analysis of the 1850 census; although, I actually read Richard's age as 51 which would make him even younger. Ancestry has interpreted his age in the 1860 census as 61, but that could also be read as 67!
In the 1830 U.S. Census of Hawkins Co., TN, Richard is listed as 30 and under 40 placing his date of birth between 1790 and 1800. Benoni's age is given as 20 and under 30 putting his birthdate between 1800 and 1810.
In the 1840 census Richard's age is 40 under 50 [b. 1790-1800]; Benoni's age is 20 and under 30 [b. 1810-1820].
A fellow researcher swore that he had positive proof that Benoni's birhdate was 9 Dec 1800; he only had 1798 for Richard. He would not divulge his source for the birthdate. He told me it existed and that I should find it. I haven't and this researcher has since died. When I get to Indiana this summer I will see if he left his records to a library or if he has a surviving relative who might have them.
Richard, Benoni, Evan, William, John and Absolom are traditional names used in every generation from the 1600's on, unfortunately, the pattern doesn't keep those names in any particular order.
By the way, I had interpreted the name "Jane?" as Susie. Ancestry reads Benoni's name as Bneng in the 1830 census; it has been interpreted as Bening; I read it as Benony. Interpretation seems as important as facts especially if there is a smudge, misspelling or a child giving the age of a parent.
Can you make any sense of this and draw a solid conclusion?