[mcguire]

As far as I know, it depends on what you mean by trebuchet. Counterpoise trebuchets (using heavy weights) are definitely medieval.

[Retric]

Yes, the giant weapons most people think of when they hear the term trebuchet is medieval improvement allowing people to further scale up the design. A mangonel (traction trebuchet) is the older design, but trebuchet is referring any scaled up staff sling.