Only personal limited observation, but I more or less discount length, because sharpness, IMO, trumps the need to be overly concerned about length, because it speaks for itself that as sharpness increases it is as a result of increasing length, and therefore age. In other words, if its cone shaped and not sharp its a 2 year old, if it has a sharp point its 3 and if its very sharp and started to curve its 4. I don't know beyond 4(the few, if any wild birds that are beyond 4). That's good old boy criteria, but I think a fairly accurate way to age them. Certainly a good enough way for us non-biologist types.