How long has agave nectar been around?

This site has some info (certainly not completely definitive though). It says: “Early inhabitants of the Americas discovered the many properties of the Blue Agave plant thousands of years ago. The plant was harvested for a variety of uses, including food, drink and clothing. It was not until recently, however, that the nectar from the plant was recognized for its exceptionally sweet properties.”

Another site indicates the commercial scale production started around 2002.

Hope that helps a little.