How young is typically “young”?
The Fields medal also has an age cutoff of 40 (Andrew Wiles being the most recent example of someone who missed the age cutoff for an otherwise sure award). 

Grant agencies usually define "young" in terms of "years since Ph.D", or "years in academic position" with the typical cutoff being 6 or so. 

Note that while for most things*, 40 might not be considered "young", in many disciplines you might only get your first academic position near that age (especially in biology). 

  * politics being the other venue where 40 would be considered young :)