I don't know the exact age for that. But on one of my later appointments of the 3rd trimester my baby was found to be in the breech position. I was so stressed out! My friend said in her pregnancy that she could feel exactly when her baby had turned, from its movements. I got even more stressed because I never felt this with my baby even though I was trying hard to 'listen' to my body and the baby. Then on a subsequent appointment, my doctor noted that my baby had turned to head-down, and I'd never felt it or known. So, just keep up with your doctor's appointments and follow his or her advice and directions, and try not to stress.