When does matchmaking kick in?

My friends and I have been playing together for a few months now. The four of us are all between level 25 and 30. I am ranked bronze 3 so I'm not that good. But we are consistently paired up against 2 or 3 silver or gold players each match. We have won only 3 out of our last 15 matches played. That's only 20% win rate.

I thought that Riot's goal was to get people adjusted so they win about 50% of their games.

When does the matchmaking system kick in and lower our MMR so we aren't constantly losing? I'm about to stop playing league because of this. loosing 10 games in a row really takes the fun out of it.