Let's say there's a Gardevoir and a Landorus-T.
Gardevoir's trace copies Landorus' intimidate. Will intimidate activate, knowing that intimidate activates when the user enters battle?
Generation IV Intimidate now also activates when a Pokémon gains the Ability, such as via Skill Swap or Mega Evolution.
source This should mean that tracing intimidate makes intimidate activate immediately in every generation after gen 3.
Yes, it does.
Source
Look at this replay. Landorus and Gardevoir come out, and Landorus Intimidates Gardevoir. Gardevoir Traces Intimidate and Intimidates Landorus in return.