Osteopenia is low bone mineral density that can lead to osteoporosis if it becomes low enough. To find out if you have osteopenia, you can have your bone density checked. Bone density is bone thickness. Your levels of calcium, phosphate and other minerals need to be measured. Another way is to have your bone mass measured, which tells you how much bone you have.
We reach our pinnacle in terms of bone mass at around thirty years of age. From that point on, bone mass will decrease.
It's recommended that women who are age 60 or older to have bone density scans, if they have risk factors, e.g., low body weight. For women without risk factors it is suggested that they have bone density scans starting at age 65.
"There are no clear guidelines for when to begin screening for women between 60 and 65 who have no other risk factors. Nor are there specific guidelines for women under 60 who have additional risk factors."
http://www.webmd.com/osteoporosis/osteopenia-early-signs-of-bone-loss?ecd=wnl_hbn_061410