In US college athletics, what is the oldest athletic conference?

The Michigan Intercollegiate Athletic Association (MIAA) is America's oldest collegiate athletic conference.

The 2008-09 school year will mark the 122nd consecutive year of operation for the NCAA Division III affiliated conference. The MIAA was founded March 24, 1888.

https://www.miaa.org/about_the_miaa/history/index

Big Ten Conference
is the United States' oldest Division I college athletic conference. Its eleven member institutions are located primarily in the Midwestern United States, stretching from Iowa and Minnesota in the west to Pennsylvania in the east. The conference competes in the NCAA's Division I; its football teams compete in the Football Bowl Subdivision (FBS; formerly known as Division I-A), the highest level of NCAA competition in that sport. Member schools of the Big Ten also are members of the Committee on Institutional Cooperation, a leading educational consortium. Despite the conference's name, since Penn State joined in 1990, there have been 11 schools in the Big Ten, as signified by the hidden "11" in the Big Ten Conference logo (each "1" is on either side of the "T" in "Ten").

https://en.wikipedia.org/wiki/Big_Ten_Conference

http://www.articlesbase.com/sports-and-fitness-articles/the-big-ten-a-premier-academic-and-athletic-conference-593356.html