I agree....15 months is pretty early. Honestly, kids will only potty train whey they are ready. I know some kids do have sphinter control at 15-18months, but if they are soiling themselves in the diaper and just don't care, then they aren't ready.
It's really more of a headache for you to push a child into toilet learning when they aren't interested.
This seems really early. I would wait until she is atleast 2. If she trains too early she could regress when she is 3 or 4 and then it will be harder to train her then.
My daughter started training at 15 months. She was interested so I let her. However, I don't push her about the subject. I take her when she first wakes up and she always uses it. And I'll ask her if she needs to use it and if she goes in that direction, I help her. She can't pull her pants up and down so I'm not working too hard at it. I figure I'll let her take the lead and do as much as she wants to and not turn it into a battle. It will make things easier when she is ready to get serious later on.