Do underweight men and women have low bone mass ? and could you get osteoporosis in a younger age? im 20 years old

Yes: Calcium is recommended early on to avoid bone loss that starts as early as adolescents in some patients.

Cause?: If you are underweight because of anorexia or severe malnutrition from a medical condition such as malabsorption then you could have low bone mass and develop osteoporosis. If you are a lean person, eating healthy, exercising and very fit, you could be underweight without any risk to your bone health.

Tricky question: Hi. Yes, young people can certainly have bad bone health due to genetic or nutritional defects. However, you don't reach peak bone mass in life until about age 30, so environmental factors (nutrition, vitamins, etc...everything but genetic factors) could still be addressed and corrected with gain of bone mass & strength. You need adequate protein nutrition, calcium, and vitamin D, etc. for bone.

Not always: sometimes underweight do, especially in chronic disease. Younger people sometimes do get osteoporosis if other disease process going on