A pedeatrician once advised a friend of mine who was worried about her baby not having done what he"should" have according to the baby books. He then asked her if her son had ead the books? She of course said no, thinking the dr was odd but he then said so if your son hasn't read the books how is he supposed to know that he's not doing things at the right time??
My point here is simply that some children walk, talk, sit etc very early & others very late, this may or may not be an indication of iq & general ability. There are so many reasons to explain why we do learn things when we do rather than when we should according to some "authority" on the topic!
Both my children are gifted but in hugely different ways, my son at 4 (now 9) could write better than his sister can now (she's 15 & about to start college/university) but she also couldn't tell the time until she was 14 1/2 years old, it simply wasn't relevant before then but when she was going to be away from home on a student exchange she knew she needd to be able to read a watch & ovrenight (literally) she could. So sometimes we do not bother learning things until we need them.
If your child is healthy, eating well & not too far behind (although my husband never bothered speaking until he was 3 & then he just started with sentences!!) don't go straight to worry mode, think about your child & then if you feel it's appropriate get it checked.