Good question Kubur. I have been wondering about that myself. I am far from an expert, but can't resist adding my two cents worth. I think the basic style has evolved slowly over time (except for the Royal Khanjar), making age determination a challenge.
I have several Jambiya that have been vetted as early 20th, including one that I think may be much older. They differ only slightly in appearance until you examine the blade. Then the difference is striking. The knife I am referring to is a Zabidi style (
http://www.vikingsword.com/vb/showthread.php?t=20476 ). I have shared photos of it with several people who I consider experts and they all concluded it was early 20th. The blade appears to be the result of a primitive forging technique. None of the other early 20th Jambiya I have handled have a blade anything like this.
I assert that the basic style was a slow evolution and that the blade is the key to identifying the older ones. I am often wrong though so take that for what it is worth!
Harry