I am not sure there is a magic age that the risk goes away; if I were you, I would research 'safe co-sleeping' on the internet.
The Science of Parenting by Margot Sunderland helped me in my quest to ensure I kept both mum and baby happy whilst as safe as possible and really put my mind at ease.
Just a quick thought, have you tried putting the pillows directly onto the mattress then putting the bed sheet over the top and tucking it in so they are there, but cannot possibly go over baby?? Or, you could sew the pillow cases to the sheet so they cannot move about? (I'm going to do this for my next one if they want to co-sleep now I have though of it!! I did without a pillow until I thought it was safe to re introduce around the one year point with my first)
My first child co slept from an early age, much earlier than I would have liked, due to him wanting excessive night feeds that I could not keep up with. My second, on the way, I hope will sleep better, but if not, I would co sleep again (though my hubby wants to wait till they are a year old, and I do agree but dont know if it will be possible!).
I feel, that if I make things as safe as possible, the risk is outweighed by the benefit. Where you add the risk of suffocation, you reduce the risk of cot death.
My husband WANTS our second child to join us in our family bed from one year old as we have enjoyed and still enjoy co sleeping with our first, who is now three, so much!! It feels so much safer once they are over one year old. I have many precious beautiful memories stemming from co sleeping, there is nothing more amazing than looking into your two year old's eyes, them kissing you and then nodding off peacefully with their arms around you neck!! Much nicer than tantrums and cries if they dont like to be alone in bed.(not for everyone, but certainly for us!!!)