How old does one have to be to enter the best young rider competition in the Tour de France?

Explanation

The white jersey is one of the jerseys in Tour de France. The best-placed rider under 25 years can wear the white jersey after each stage. The youth competition was introduced in 1975, and has been won by several legendary Tour-winners such as Laurent Fignon (1983), Greg LeMond (1984), Marco Pantani (1994, 1995), Jan Ullrich (1996-1998) and Alberto Contador (2007). There was no white jersey 1989-1999.