My 17 month DS clearly tells us 'poo' when he needs changing. My MIL told me he's ready to start and if I miss the window of opportunity he will probably not learn until he's around 3.
Id not heard this before. My husband then read in some parts of the world babies are taught from 6 months, when they can sit. Just by being placed on the potty.
I don't want to rush this. We are expecting DC2 who will arrive just after DS 2nd birthday, so ideally training doesn't directly clash for obvious reasons.