I'm not sure if I should be worrying yet or not about this, but my DS is almost 17 months and still doesn't have any words apart from "mum/mumma" (and not in context either.) He does understand certain requests and instructions (please pass mummy your dummy / can you put it in box / where's your ball etc)
He doesn't gesture very much, if at all. Can point and wave and clap, just never ever does! Doesn't always respond to his name either. He's super active and into absolute everything.
We do lots of baby groups, songs, reading books, modelling clear language, back and forth games, etc. We also use very basic makaton signs but he's never tried to use them himself.
At what age would you flag it up? It doesn't help I'm on groups of Facebook where lots of the toddlers are now saying and mimicking lots of words, I can't help but compare 
(which I know is the worst thing to do!) many thanks.