From the Core set:

Boar Boys (ORC) or Squig Herders have this ability: "This unit gains 2 (or 1) attack as long as you control 1 damaged unit".

So what if my Orc units are undamaged when enemy is attacking and I'm declaring defenders?

Once they attack, say some of my units are not killed, and are now "damaged." Do Boar Boys get +2 immediately, so they can deal 2 extra attack to the enemy this same round? Or do I have to wait till my turn starts to have this effect trigger?

Thanks!

You do not have to wait until your turn starts. These are constant effects, they are active as soon as and while the condition is fulfilled. But there's only one step where combat damage is assigned. Only the power of the units at that time counts. So both Boar Boyz and Squig Herders gain power as soon as they are damaged, but the amount of damage they deal was determined earlier and doesn't change retroactively.