How young is typically “young”? I was reading about a “young researcher award” recently being attributed in my field to a colleague who is 40 years old (and has had a permanent research position for 12 years now). It may be viewed as borderline ridiculous, so I must ask: in this context, how young is typically “young”? For the purpose of this special projects/grants/awards, how do research and funding organizations define “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 :)