both of my twins did everything later than all of my older children~
My DS did a commando crawl but not even until after he was one! I also remember that when he was 11 months old he still wasn't sitting up on his own! I was always a little concerned but their Dr. never was(thank God) they both walked late to me! DD was almost 16 months and DS just shy of 20 months!
I think alot of it has to do with always putting them on their tummies vs on their backs!I have 3 older children and the latest one walked at 14 months!Also my Dr. would say because they are twins and I couldn't possibly give them the attention I could with just one~
I decided to quit comparing and just let them go at there own pace~
They are now both very active toddlers and I never really think about when they walked or crawled etc.
I wanted to add that DS still seems kinda rubbery to me but again Dr not worried so puts me at ease~ HTH's