When should one use the Revolution age in AOE 3?

Using Revolution is in essence you going "all in" (in terms of poker).

The best times to use this is when you want that extra little boost to tip the scale in your favor. This scale tipping doesn't last very long and is rather risky, but if you take advantage of it, that's all you'll need to win the game. There are a couple things that could spell doom for you however:

  1. You lose the proceeding battle: if you lose your army, you will have a very difficult and slow time getting it back unless you've saved a large amount of resources or have a solid income of XP for shipments.
  2. Your enemy advances to the fifth age: There are many technologies that drastically improve the military and economy in the fifth age that are impossible to obtain once you revolt.

I usually only use revolution when I don't have the time/resources to advance to the fifth age, and I'm also pretty sure that my enemy doesn't have time either. If I'm right, it usually spells victory for me. If I'm wrong, it usually spells defeat.