If a Unit becomes active before the End of Turn phase, it can participate in actions that require it to be active during that same turn.

This is supported by the following rules:

• On page 26, it states: "All your Units become active (Rotate all exhausted Units to a vertical position)" during the End of the Turn phase, but it does not state that Units cannot become active by other means before this phase.
• On page 24, during the Attack phase, "your Units in attack position start the offense towards the enemy Sanctuaries, from left to right." There is no restriction mentioned that prevents a Unit that has become active by an ability from attacking or defending in the same turn.
• On page 21, the ZEALOUS attribute is defined as: "This Unit is deployed active (not exhausted) on the battlefield." This implies that being active allows immediate participation, as there is no mention of a waiting period.

Therefore, if a Unit's ability allows it to become active before the End of Turn phase, it can attack (if in attack position and during the Attack phase) or defend (if in defense position and attacked) during that same turn.

Source: pages 21, 24, and 26.

Source: Core Rulebook (pp. 15, 21, 24, 26, 33)