I know my nephew didn't even start crawling until he was 17 months, and walking was a few months off that - so it can be normal to not have started walking by now.

DS and DD1 both walked at 15.5 months - so thats only 2 weeks difference to your DD

DH heard from someone (a podiatrist I think?) that kids will only start walking when their feet have flattened out... are hers flat on the bottom?