I have been thinking about this saying a lot in the past week (and yes I saw Prince in concert 30 years ago, and the Ramones the same night), but I have heard it since I was a child. I guess I find it amusing because my European shoe size is 48 and I have only been older than that for a few years. I finally grew up, apparently!
So, where did the saying "Act your age, not your shoe size" come from? US? UK? Can't have been from EU, unless they changed their shoe size units recently.
Best Answer
The earliest match for "Act your age, not your shoe size" that a Google Books search finds is one from 1967 cited in Charles Doyle, Wolfgang Mieder & Fred Shapiro, The [Yale] Dictionary of Modern Proverbs (2006):
Google Books confirms the Schoen citation as being from a novel published in 1967.
A related anti-proverb listed in the same dictionary is "Act your age, not your IQ," for which the dictionary gives a first citation date of 1995. A Google Books search, however, finds an instance of this expression in Student Lawyer, volume 16 (1987) [combined snippets]:
Another early instance of "Act your age, not your IQ" appears in Jolene Prewit-Parker, Homecoming (1990).
A Google Books search also turns up multiple instances of "Act your age [and] not your color," dating back to Clara McLaughlin, The Black Parents' Handbook: A Guide to Healthy Pregnancy, Birth, and Child Care (1976) [combined snippets]:
All of these expressions appear to be extended variants of "Act your age," which Jonathon Green, Cassell's Dictionary of Slang (2005) identifies as originating in the United States:
The earliest search result matches that I've been able to find for "Act your age" are from Life magazine, volume 85 (1925) [text not shown in snippet window]:
and from the [Brownwood, Texas] Yellow Jacket (October 13, 1926):
Both of these instances are indeed from the United States.