Does anyone know when babies start getting the idea of what shaking or nodding your head means (ie no or yes)? When did your baby start doing this?

I'm curious as Kynan's just started shaking his head at me when he doesn't want anymore food. I'll pick his spoon up out of his bowl and ask if he wants more and he shakes his head (it's very cute LOL!). I'm thinking that he's perhaps picked this up from me as I've recently realised that I shake my head when I say 'no more?' when I know that he's full.