I heard my daughters at 6 weeks.
At 16 weeks my doctor tried for the first time and couldn't find it. Had to use an ultrasound
I didn't hear my baby's heartbeat until about 12 weeks. Something about too much strain on the baby, and it being hard to find.
The drs heard my first at 10w but they said it was really early and warned me that if they couldnt find it not to be alarmed
I had access to a hospital doppler with my first son and I started looking for the heart rate at 12 weeks and I could find it. There is a big artery in your stomach that is easy to pick up so if the heart rate seems slower than it should you probably are hearing yours. I used to drive myself crazy trying to find it because I was so afraid of something happening to my baby.