If my opponent kills one of my blocking, say, Young Wolves (undying 1/1). It immediately becomes untapped and gets a +1/+1 counter on it. But I understand it has summoning sickness ( since it changed zones).
So, now it's my turn. Does this creature still have summoning sickness? Or does that end of any player's turn?
No it does not have SS for it is the second turn u own him
Creatures have summoning sickness if you haven't controlled them since the beginning of your turn. In this case you did, so they can attack.
If they {Shock} your wolf on your turn before combat, you won't be able to attack.