This information can be had in statistical fashion -- ie., at a given date, X% of pieces on programs of this type (orchestra, salon, etc.) were by living composers and X% by dead composers -- from the work of William Weber. See The Great Transformation of Musical Taste.
I assume you are not talking about church music, either, since Gregorian chant (ca. 900) has been performed every day since the Middle Ages and continues to this day.
Leaving out church music, we'd have to start around 1600, when secular music really begins to have its own history; here's a rough guesstimate of what your function would return, based on my study of music history over the years:
1600 - 1600 (monody displaces polyphony in secular music)
1650 - 1600 (first operas do not disappear right away)
1700 - 1600 (still some Monteverdi being performed...)
1750 - 1725 (Galant style takes over)
1800 - 1750 (no canon yet; Mozart dead and buried)
1830 - 1700 (Beethoven becomes first "immortal" composer; Bach revived)
1850 - 1700 (same)
1880 - 1750 (orchestra culture dominates opera in USA; more focus on classical and romantic music)
1900 - 1750 (same)
1930 - 1800 (narrowest definitions of "great music" center on Beethoven)
1950 - 1600 (baroque revival)
1960 - 1500 (early music revival)
1980 - 1500 (height of early music movement)
2000 - 1500 (same)
2015 - 2015 (Taylor Swift displaces all previous music, becomes sole inheritor of Western Art Music tradition)
UPDATE: The following is a graph of the maximum age of performed music, throughout the times, according to Robert's data:
