My 17 month old DS can't talk at all - no discernible words whatsoever, not even mama or dada, no animal noises etc. He babbles a lot but 0 words in context. He waves, babbles, shakes his head for no and seems to understand what we are saying but no words at all. He's a very physical child so just wants to be zooming about all the time. My older DS was slow to talk but he definitely had a few words at this age. Should I be worried? I'm generally fairly laid back but I would have expected at least one of mama/dada/hello/no/bye at this age.