katedan, I think you will find it isn't very unusual for a child to walk later than 16 months. More rare later than 18 months, and very rare indeed later than 2 years if there is nothing wrong.
There's absolutely nothing wrong with my younger son, who walked at 17 months - he's actually rather physically able. My other son walked just before 23 months and he has low muscle tone (no diagnosed cause - there frequently isn't...), but his grandfather walked after the age of 2 and had no motor problems whatsoever in later life - he was, in fact, a very good tennis player and no-one could beat him at billiards...
Did your DD have trouble with her other gross motor skills? Apart from being a bit on the late side with the early ones, my younger son didn't, but it was just so obvious to me from a very early age that his older brother did have a problem - he was obviously bright in all other ways, but couldn't even roll over at 15 months, let alone get himself from lying to sitting, bottom shuffle, crawl, pull to standing, etc. Even with that rather troublesome start on the gross motor skills front, my DS, now 4, can skip and jump and run - the only things he still finds a bit of an effort are cycling up a slope and climbing stairs without the help of bannisters.
So, please try not to worry about the more serious possibilities (I know it's difficult - I used to cry myself to sleep, worrying about my eldest DS and why he couldn't get about, despite his obvious desire to). Usually, it turns out to be something that in the long term doesn't add up to anything that serious.