I have found that all kids I know have their own late starts, just depends on the kid as to what that might be My own DS didn't sit until he was about 7 months old, used to really stress me out. He just had no interest, wouldn't even tolerate being propped up with cushions! He also didn't ever commando crawl which worried me until one day he just took off across the room! He didn't sleep through until he was almost one year old.

But on the other hand he cut his first teeth quite early and walked before he turned one, again just took off one day! Some kids I know older than my DS haven't stood on their own or said their first word yet, but slept really well from a young age or did something else early. It's really interesting to see how everyone is so different.