Your baby should at least attempt to crawl by the age of 7 months. Each baby develops at his own pace. You can encourage your baby to crawl by putting him on tummy time and placing a toy just a little out of his reach so he will move towards it. Some babies skip the crawling stage altogether. I was one of them. When I moved about on all fours, my knees never touched the ground. http://sg.theasianparent.com/baby-milestones-development/ http://www.babycenter.com/400_how-can-i-encourage-my-baby-to-crawl_500136_0.bc
It differs from different babies. Like Diana said, most babies start at 7 months old but my son who's 6+ months still shows no instead in crawling. He just lays there and flips back and forth before demanding us to pick him up so that he can "stand".
Baby will start to crawl around 10months. However, some babies do crawl earlier than 10 months or later than 10months. If you worry about your baby's development milestones, please visit PD as soon as possible for a detailed check.
Each child has it own pace. My child started crawling when he was 11 months old. By my bestie's child started at 8!
thanks