Crawling

Chrysta

What ages do they start to crawl? I see a lot of 6 month olds on here crawling and mine is 6 1/2 mo the and is not. He will scoot backwards but doesn't get on his knees. He doesn't even roll over back to tummy (rarely tummy to back) the dr wasn't concerned at all because he sits up well (can't get into that position tho) so I feel he is behind but not sure how behind.