"You said it” meaning “I concur”.

How old is this? For some reason I can image Dean Martin saying it in the 1950s.

I can imagine seeing it in an early 20th century comic strip.  Is that a significant antedate?

Well, touche…

Should I have used a winky? ;)

The OED has it from 1919, and that first entry is from Jargon Book by C.H. Darling, evidently a compilation of slang, etc., so it was widespread enough at that time to have come to the notice of the compiler.