@Kat919 - I'm glad you pointed that out. It's another reason why we need to keep young people moving, since they are building their bones at that age. Get them off the couch and into whatever they like - running, karate lessons, etc.
A friend of mine who is a vegan told me something interesting that people might not realize. Apparently, eating high levels of animal protein (as from meat and dairy) causes your body to *lose* more calcium in your urine than you otherwise would. The calcium recommendations from the USDA are very high to compensate for that.
So vegans and vegetarians may not need as much calcium as omnivores, and calcium from vegetable sources is fortunately just as well absorbed by the body. If you have a teenager who goes through one of those vegan phases (and you never know, it could stick like it did for my friend), educate him or her about broccoli, dark leafy greens, etc. to make sure s/he gets enough calcium.