I really don't think 15 months is young at all. I mean, sure, by the "traditional" (for this decade) keep-them-in-disposables-until-they-can-ask-in-full-sentences crowd it's young, but developmentally it's not early for some.
We started around 14 months with a toilet seat and, having done BLW, we're doing BLPT. We don't have the expectation that we'll be out of nappies any time soon, but because we're using a toilet and not a potty, every time he goes in the toilet is one less nappy and that's got to be good, right?
From about 13 months they start processing sequences and routines are suddenly comprehensible for them. Mimicking is another developmental leap around this time that helps if you let them see you use the toilet. Around this age, our DS started asking to feed the dog every morning. Well, I though, why can't he ask to go pee too? And he can! We started out with first thing in the morning. He used to fill two nappies on the trot; now he wakes up, has a lovely long cuddle and breastfeed, then runs off to the toilet and has a big wee, often having stayed dry since midnight and having nursed 2-4 times in between. After a few weeks he's also happy to poo on the toilet and has asked several times to go make a wee. He also responds to the question "do you need to make a wee?"
Is he toilet trained? No. Can he hold it for very long? No. Does he always know before he goes? No, but sometimes he does.
As with all things parenting, the trick is consistency, realistic expectations, watching them like a hawk, a casual attitude to "accidents" and a little bit of sixth sense.