This from Guinness World Record website; The World Record for the oldest racehorse to win n the flat was aged 19 years. 'Al Jabal', a pure-bred Arab, ridden by Brian Boulton, owned by Andrea Boulton (both UK), won The Three Horseshoes Handicap Stakes (6 furlongs) on 9 June 2002 at Barbary Castle, Wiltshire, UK.
The 17 year old in question, Victory Gunner, won as a 15 year old and has been placed a couple of times this season. Despite being pulled up yesterday (a quite rare poor run for this horse) there are no immediate plans to retire it.