DD2 didn't sit up until she was about 10months, didn't crawl until she was about 13 months and walked at around ummm 15 months I think. I was very concerned as my other 2 did everything "on time" my GP on the other hand wasn't worried at all and turns out he was right she is now a happy and extreamly active 2 year old that runs everywhere

But if you are worried then speak with someone even if just to put your mind at ease