Who needs a calcium supplement?

All. As far as i'm concerned, all could benefit from 800-1200 mg daily. Broccoli and salmon has more calcium than a glass of milk and more nutritious too. Children and post menopausal women are at highest risk of poor bone health. Vitamin d (2-5000 units) is also needed for bone health.
Vitamin D too. Shoot for levels at least 50 for optimal health . We think it is probably a hormone and not a vitamin and just now learning the role in immunity, heart and colon.
If not enough diet. Need about 1200-1500mg calcium: all sources (diet and supp), about 800 iu vit d/d. Takes work to get it .Dairy (low fat/no fat) easiest (200-250mg/8 oz, or 1.5 oz cheese). Broccoli 75mg in 1/2c; canned salmon or sardines with bones give 200mg, 8oz tofu, 1c bok choy, collards, mustard greens give 200mg. Fortified soy/rice/almond milk can give; less absorbed than dairy. Almonds/sesame ok, but cals.