My dd has slept on her stomach since she could roll onto it. She hates being on her back. I would dare say she was 4 mo old sleeping on her tummy.
With the huge back to sleep campaign, there has been a failure to educate mothers that once your baby can move and goes onto their stomaches themselves, it is probably okay to leave them. No soft bedding or toys, etc.
I have read that not only is SIDs a breathing issue, but a heart issue as well. Some babies' hearts have stopped and they think it is from the weight of the body from being on their stomaches. I would hope by 6 mo old that if there were a heart problem, it would have been detected at well baby appts for those who still do them.
Have peace millions of babies lived sleeping on their stomaches.