Donald MacDonald (a gelding shipping from Santa Anita) was 3rd in the 1909 Belmont Stakes at age 3.
Here are a couple of cuts of his form from his 22 win season in 1913:
He raced long enough that he won a race at age 12.
I can't find his career record -- but I'm guessing he probably started several hundred times and won a great many races. 22 wins in one year is the best I've found for a horse who raced in the early 1900's.