DD#1 crawled at 11 1/2 months (and walked at 13 1/2 months). She could roll and swivel so could pretty much get to where she wanted to go, I figure she didn't really have much motivation to crawl because of it.
All babies do things at their own rates. People always seem to expect a baby to be walking by the time they are one, but the average age to walk is 15 months! I know a lot of babies are walking earlier, so some must walk heaps later to make the average 15 months!! I guess it is much the same with crawling. I'm hoping DD#2 doesn't start crawling really early, I like having her stay where she is put (for the moment), I have enough to do chasing DD#1!!