In conditions of growth or when calcium is low in the diet, a Vitamin D based calcium transport system is activated in the body that in effect increases calcium uptake.
This mechanism however can decline with age and menopause in women. Sodium and caffeine can also increase calcium loss while excessive zinc or magnesium (supplements) along with phytates and oxalates (spinach, rhubarb) can reduce absorption.
The boy in the photo is also very low in zinc, as can be seen by the white spots on his nails. He also undoubtedly has other nutritional insufficiencies.
Calcium RDI’s for Australia