Supercross races first began in 1972, but 1974 was the first year that Supercross was a full series.  Pierre Karsmakers won that first 250 championship.  The 250 class is now called the 450 class.

For the first 11 years of Supercross, there was no 125 (250 four stroke) series, with the first 125 series being introduced in 1985.

Starting with Karsmakers, there have been 20 different champions in the premier class.  For the most part, these future champions were quite young when they joined the premier class.  Every champion from 1983-2016 was no older than 21 when they joined the class.

The last future champion to join the class older than 21 was Donnie Hansen, who won the championship in 1981.  Hansen was 27 when he first joined the class in 1979.

Here is the age of each champion when they first joined the 450/250 2-stroke class:

Interesting find: In the 2014 Supercross race at Detroit, the first heat race in the 250 class features 17 riders on Hondas, out of the 20 total racers.

Gavin Faith won the race and was one of 8 Hondas that transferred to the main event.