It is fairly common these days but it should not be considered normal. It has to do with a lack of nutrition or a lack of absorption of nutrition. Food today has so many chemicals (pesticides, preservatives, hormones, antibiotics, etc) in it that it breaks down the nutrients so we can't absorb them.
Both of my girls are on an absorbable multivitamin/mineral complex that has a guarantee for absorption. My 14 year old had an odor at seven but doesn't have one now. It's the nutrition and the body working the way it should. If you're interested in what I did, let me know. I'd love to help.