What standard deviation below the mean of normal young adults equals osteoporosis?

The standard deviation from the mean of a young (30-year old) adult is called a t-score.

If the t-score is - 1, it means that the bone density is 1 standard deviation below the mean.

It is generally considered a t-score between - 1 and - 2.5 low bone density.

Patients with t-scores below - 2.5 (e. g. - 3) is considered suffering from osteoporosis. Also, patients within this range AND suffered from one or more fractures is considered established osteoporosis.