For single season leaders, statistics from the National League era (1876-2016) are counted. For career leaders, the National Association years (1871-1875) are also included. (This distinction only affects Cap Anson's National Generation. Every player from the eight succeeding baseball generations, with the sole exception of one Bill Crowley, debuted in 1876 or later anyway.)
For single season rate statistics, only those seasons where the player qualified for the batting title, ERA title, or winning percentage title are included. Currently, the standards are 3.1 Plate Appearances per team game (for the batting title), 1 Inning Pitched per team game (for the ERA title), and 1 decision for every 12 team games (for winning percentage). But the standards have changed over time. Here is baseball-reference's list of standards used.
For career rate statistics, the minimum numbers to qualify are 3,000 PA, 1,000 IP, and 100 decisions. For SB%, 20 stolen base attempts (for single season leaders) or 100 stolen base attempts (for career leaders) are needed to qualify.