"Achieving adequate calcium intake and maximizing bone stores during the
time when bone is rapidly deposited (up to age 30) provides an important
foundation for the future. But it will not prevent bone loss later in life.
The loss of bone with aging is due to several reasons, including genetic
factors, physical inactivity, and lower levels of circulating hormones
(estrogen in women and testosterone in men)."

Source: http://www.hsph.harvard.edu/nutritionsource/calcium.html 

Parathyroid tumor can cause osteoporosis that's reversible when the
tumor is removed.  See the link for more info:
http://www.parathyroid.com/osteoporosis.htm

In your shoes this would be the diagnosis I'd want, as it has an easy
fix.  Parathyroid surgery is no big deal -- meaning no overnight
hospital stay for most people.

Sedentary lifestyle, smoking, and copious caffeine intake also increase
risk for osteoporosis, and some medications, especially corticosteroids
like prednisone.  Most of the other risk factors aren't controllable,
like family history for example.

Do you have any of the risk factors listed above?  Have you had your
parathyroid level checked?  What has your doc told you?

--Patti