Hi ckl,  My son is under an endocrinologist and a metabolic bone specialist because of low bone density.  They do not advise calcium supplements but recommend calcium rich foods instead.  Peak bone mass is not reached until the late 20’s, so kids have plenty of time to build up their bone density before this and generally as they go through puberty and produce higher testosterone levels, their bone density will improve naturally anyway. 

Intervention by way of supplements is not generally recommended until a person is at least in their mid 20’s and it looks as if they may need help in reaching a normal target range.  I was told it is far better to try and reach this naturally though than to rely on supplements at too an early age.  Of course different specialists may have different views - this is just what I was told!