WAY longer than that. What's come to the forefront specifically in the last year or two is very specific subset of the overall agent landscape. What I like to call "LLM Agents". But "Agents" at large date back to at least the 1980's if not before. For some of the history of all of this, see this page and some of the listed citations:
https://en.wikipedia.org/wiki/Software_agent
> Agents are just LLMs with structured output
That's only true for the "LLM Agent" version. There are Agents that have nothing to do with LLM's at all.