What year did the colts start playing in Indianapolis?
1970.
When the AFL-NFL merger took place in 1970, there were 16 teams
in the NFL and 10 teams in the AFL. To make the number of teams
even in the newly created conferences, three teams from the old NFL
(new NFC) moved over to play in the AFC. These teams were the
Colts, Cleveland Browns, and Pittsburgh Steelers.