from Sep 19, 1951 HH:
Among the starters at the Lyndonville, Vt., meet was the 22-year-old Worthy Symbol, who made six starts over a two day period. In one of Worthy Symbol's races, the average age for the starting field was 14-1/2 years.
from Nov 11, 1930 Trotter and Pacer:
B. C. Mayo, owner of Piney Grove Stock Farm, Tarboro, N. C., has shipped the yearling colt Worthy Symbol, by Symbol S. Forrest-Miss Austin, by Ortolan Axworthy, to Dr. H. M. Parshall at Urbana, Ohio. The colt is a good sized and well made individual, with nothing but pace in his head, the color of
his sire, and is believed to be an exceptional prospect.
end of quotes
I don't recall when the 14YO Rule was implemented (15YO's used to be able to race at Fairs and still may be able to do so), however the 1969 Ainslie Book on harness Racing had the USTA Rules published in the back of the book, and the 14YO Rule was in effect then.
Why doesn't somebody
Ask the USTA