Which player had the longest professional career?

Wilfred Rhodes played test cricket for nearly 31 years. http://www.espncricinfo.com/england/content/player/19376.html

There are a significant number of athletes who have professional careers longer than 24 years. Going for the longest by sport:

• Basketball: Robert Parish, 21 years
• American Football: George Blanda, 26 years
• Baseball: Cap Anson and Nolan Ryan, 27 years
• Ice hockey: Gordie Howe, 33 years, including 1972 when Howe didn't play. If Howe's one shift for the Detroit Vipers in 1997 is included, then his career spans 50 years.
• Cricket: The longest first-class career was possibly WG Grace, 43 years from 1865-1908. The longest Test career was Wilfred Rhodes just over 30 years, from 1 Jun 1899 to 12 Apr 1930.
• Golf: Gary Player, 56 years. Player turned professional in 1953 and competed in his last professional tournament in 2009.

Sources: The Roosevelts Longest Sports Careers, linked Wikipedia and Cricinfo articles.

The challenge is defining 'sport'. The more physical the sport, the more likely for it to be considered a sport, but the longer careers would naturally be in the sports that require less stamina and strength. These also tend to be where some would not call them a sport.

• bowling: Pete Weber, 35 years
• poker: Doyle Brunson, over 50 years
• snooker: Steve Davis, 46 years
• auto racing: Larentiu Moldovan, 1 day shy of 40 years
• darts: Billy O'Brien, 34 years

I can't say I follow any of those particular sports too intently. I can't say if those are the longest careers in each one, just that they appear to be notable player with notably long runs.