My son is 18 months, and for the last three months he has been telling us when he needs a poo/wee and when he has done one. He even gets the change mat, and nappy bag and putsit on the floor in front of the fire and lies on the mat.
He is dry for 2-3 hour periods in the day and happy to run round in the nuddy.
He knows to sit on the potty but just won't anymore (even for reading, sweets, stickers) if he does sit or is put on it he jumps up immediately and laughs. It's not cold because it's next to hearth.
He's advanced with language, saying over 80 words and understands even more but has been getting quite frustrated lately with a lot of things. I just wasn't doing something right.