the stamina to achieve consistent top ranking results, comes from a selection of coincidences, urge, ability, timing, luck, training, and above all refusing to give in to tiredness, fear, pain, or the competition.
This pretty much sums it up. Racing since 4 years old. Copy and pasted from wiki
Early Biking Career
Aaron Gwin started his cycling career off early racing BMX from age 4. By age 8 Gwin was racing all the nationals. At age 12 Gwin moved on to motocross and raced up until he was 17 when he quit due to constant injuries. In 2008 at age 20 Gwin was loaned a bike by fellow professional downhill racer and good friend Cody Warren and was encouraged to compete in a race. In his first race in the Fontana Winter Series he got 3rd and began racing from then on, quickly signing with Yeti Cycles.[3] He has since won two United States National Championships for Downhill, in 2009 and in 2010. Both were held in SolVista, Colorado [edit] Riding Style
Much of Gwin's style and skill come from his motocross days, and his whips and jumping technique show it in videos. He also gained a lot of flow from racing BMX for so many years, it also shows in his Dual Slalom and Mountain Cross results. (2011) Aaron is currently team mates with Justin Leov,Tracey Mosely, and Neko Mulally . Aaron joined the Trek World Racing Team in November 2010 after relocating to Temecula, CA.
Watching his section on 3 Minute Gaps it's pretty obvious of his MX background. A lot of the more talented (and quicker) motorcycle road racing lads are ex-schoolboy MX champs too. Seems you can go either way when it comes to using MX as a starting point in your youth.