I read posts where DC are following all manner of instructions at 16 months - going and fetching, doing things for themselves etc.
My DS definitely understands stuff - but I can't really tell how much. He'll understand go and get ball, pass me a bit of your biscuit or do you want a banana, do you want teddy, milk etc etc. I suppose in day to day terms, I assume he understands what he needs to - and he's very communicative (non-verbally).
But some of the posts have worried me.