8-10 months. In order to crawl you must be able to sit and have strong back and leg muscles, some babies that do not get enough tummy time may have delays in rolling and crawling. Keep in mind many babies do not crawl and go straight to standing and walking. As long as your baby finds a way of getting around do not worry. Some babies scoot, or roll or commando crawl and never really crawl.
7-8 months. Most of full-term born infants begin crawling and pulling to stand around 8 months of age, followed by cruising. If an infant was born prematurely, it is expected he/she will achieve this milestone at later age.
6 months. Typically babies begin to crawl at 6 months. Crawling, scooting and rolling from a medical point of view are all the same thing.
8 months. Babies learn to sit first, usually after 6 months of age. They crawl a few months after that, and then start standing up while holding onto furniture.