If my baby is nine months old and not crawling yet, should I be concerned?

Not necessarially. Usually babies begin to crawl around 6 months of age. Remember that scooting, crawling backward and rolling around to get from place to place all qualify as crawling. If your baby is not doing any of these things, then developmental screening at your pediatrician would be a good idea.
Probably not. Development of motor skills in the first 18 months varies from baby to baby. Some babies seem to "skip" crawling and go right to standing and "cruising' on furniture. Have your baby's doctor examine him/her. There are many other factors to be considered before deciding whether or not to be concerned.
Probably not. Most babies sit at 6 months. Crawling comes a bit later, around 8 months. A baby who is not crawling at 9 months can see the doctor at the 9-month check-up, to recheck the motor skills development since birth, to see if there is a problem or not. Being one month late on crawling can be ok, if other skills are developing fine, and the baby is not weak nor soft in the muscle tone.
Not yet! Whereas many developmental guidelines suggest "most babies are crawling by 9 months", this is just an average age. There is actually a wide age range during which crawling develops. It is common for crawling not to occur until after the first birthday. So, do not worry just yet about a 9 month not crawling. But if there are concerns, these should always be addressed by the pediatrician.