This is described in 49 CFR Federal Motor Vehicle Safety Standards (FMVSS)
Section 571.111 - Standard No. 111; Rearview mirrors has the following:
S5.4.2Each convex mirror shall have permanently and indelibly marked at the lower edge of the mirror's reflective surface, in letters not less than 4.8 mm nor more than 6.4 mm high the words "Objects in Mirror Are Closer Than They Appear."
Section 571.1 "Scope" informs the reader:
This part contains the Federal Motor Vehicle Safety Standards for motor vehicles and motor vehicle equipment established under section 103 of the National Traffic and Motor Vehicle Safety Act of 1966 (80 Stat. 718).
(33 FR 19703, Dec. 25, 1968. Redesignated at 35 FR 5118, Mar. 26, 1970)
571.8 Effective date:
Notwithstanding the effective date provisions of the motor vehicle safety standards in this part, the effective date of any standard or amendment of a standard issued after September 1, 1971, to which firefighting vehicles must conform shall be, with respect to such vehicles, either 2 years after the date on which such standard or amendment is published in the rules and regulations section of the Federal Register, or the effective date specified in the notice, whichever is later, except as such standard or amendment may otherwise specifically provide with respect to firefighting vehicles.
(36 FR 13927, July 28, 1971)
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So, to answer your question: Vehicles made in September 1971 were model year 1972, and it appears that it also includes firefighting vehicles, and this is when it started.
While this is when it became effective, it may be that auto manufacturers had an "allowance window" before including the warning, but no doubt some (maybe all) car makers began it at this time.
Source:
http://www.cs.brandeis.edu/~zippy/49-CFR-571.html
