I think it has a lot to do with disposable nappies. The children just don't feel wet with them so it is easier to just wee in them. I asked my mum about how old myself and my siblings were when we were toilet trained. She told me that my 2 brothers and sisters were trained by 18 months and I took a bit longer, nearly 2. She used cloth on us.
I don't think we can know exactly when they will go to the toilet but we can watch for the signs and try to learn them. I started doing what I believe is called toilet timing from when DD was about 10 or 11 months. Just about all poos were in the toilet and most wees, which saved heaps on washing. She is now making the transition from timing to knowing when she wants to go. I also used cloth nappies, don't know if this made a difference.