Bill Davidson said:
There is some type of deed in 1880 in Gloucester Co., VA between my
maternal gg-grandfather, Smith W. Brown, and Newington Baptist Church.
The letters "TR" are used after his name, and the letters "TRE" are used 
after
the name of the church.  Can anyone confirm what these letters mean
(was "TR" actually "trustee," perhaps)?

Bill, try this but further research will likely be required.  It's a start 
anyway.
http://www.mojolaw.com/defs/ab015
(from: Bouvier's Law Dictionary, 1856 Edition.)

I was more fortunate in that the church trustees were named (and called 
'trustees') on the 1/2 acre land deed for my ancestor's 'sale' to the M.E. 
church in 1832 (for $1.)  I suspect 'TR' and 'TRE' were legalese terms of 
the times, rather than something simple like 'trustee,' as contents of the 
above hyperlinked site indicate.  Your writing infers this was not a typical 
land deed sale, is it possible your 'some type of deed' may have been some 
type of 'tenant' arrangement or indication of legal limitations (thus the 
'T' part of the abbr.?), see:
http://www.mojolaw.com/defs/te007

As to your age question, sixteen was the legal age to be taxed in several 
states in 1800, according to _The Source_, including Virginia.  However, 
there are caveats concerning tax list research as the following web site 
attests, written by a very knowledgeable genealogist:
http://www.bcgcertification.org/skillbuilders/skbld069.html
Info concerning earlier times:
http://www.lva.lib.va.us/whatwehave/tax/rn17_tithables.htm

Please let us know your findings.

Good luck,
Neil McDonald