Two different national societies (Family Medicine and Rheumatology), have specific recommendations on when to test bone density: 

Don’t order DEXA (Dual-Energy X-ray Absorptiometry) screening for osteoporosis on low risk patients.

While all patients aged 50 years and older should be evaluated for risk factors for osteoporosis using tools such as the Osteoporosis Self-assessment Screening Tool (OST) (see Quick Links section), bone mineral density screening via DEXA is not warranted on women under 65 or men under 70 at low risk.

Don’t repeat dual energy X-ray absorptiometry (DEXA) scans more often than every 2 years.

The use of repeat DEXA scans at intervals of every 2 years is appropriate in most clinical settings, and is supported by several current osteoporosis guidelines. Because of limitations in the precision of testing, a minimum of 2 years may be needed to reliably measure a change in bone mineral density (BMD). If BMDs are stable and/or individuals are at low risk of fracture, then less frequent monitoring up to an interval of 5-10 years can be considered. Shorter or longer intervals between repeat DEXA scans may be appropriate based on expected rate of change in bone mineral density and fracture risk

An excellent patient handout from the Choosing Wisely site outlines when to get bone density tests and how to keep bones strong. View it, along with more recommendations from Family Medicine and Rheumatology, in our Quick Links section.

If anyone would like to delve deeper into the evidence for bone density testing, CADTH (Canadian Agency for Drugs and Technologies in Health) provides a thorough review of the available evidence, and we've posted this in our Quick Links section too.