It really depends on the child. 15 months does sound a bit young to me, but I'm sure there are plenty of people who will say different. During my child care training they drummed it into our heads that a child doesn't have much (if any!) bladder and bowel control until about 18 months of age. Also they need to be able to clearly express to you they need to use the potty or toilet.
I started training DD 2 months before her 2nd birthday, but I then got terrible ms with this one, so put it on hold until Dec, after a couple of weeks with little success I was ready to stop for a few weeks (try again come Jan) when I just let her run around bare bummed one night and she just took herself to the potty several times, so for the next 2 weeks or so we staid close to home (only going out for short periods with pull ups on), I kept her pants off and she just trained herself pretty much. By Christmas she was in underwear with very little accdents. She is still in a night nappy, but only wets some nights, not a lot at all.

They also say after 2 weeks if you haven't got very far then it's best to stop completely and try again later as for some children if they're not ready they may feel pushed and get scared or go backwards.

As I say, all children are different, so maybe your little one is getting ready. Maybe google TT readiness checklists as there are such things! That will give you a good heads up if your DS is showing signs of being ready.

Good luck with it all, when ever you do start training.