The concept of osteoporosis as a menopausal disease did not exist before the 1970s. Before that time, osteoporosis was simply a disease of old age in some vulnerable individuals, male and female.
However, when the use of HRT declined in the mid-1970s due to early worries about cancer, drug companies hired marketing firms to promote these drugs. Small studies in the early 1980s showing that oestrogen could help strengthen bones in the short-term were used as the basis for making this claim for the longer term. Articles written by drug company public relations firms and media puppets started to appear in women's magazines, and a new market was born (Conley S, The Menopause Industry, Women's Press, 1995).
In fact, HRT does not protect bones (Am J Med, 1988; 85: 847-50). Most women take it between the ages of 50 and 60 - an age when they are not at risk of fractures. To obtain a preventative effect, the current medical advice has been to remain on HRT for five to 10 years. But research has shown that even this regimen doesn't prevent osteoporosis (and fractures) later on (N Engl J Med, 1993; 329: 1141-6).
Once HRT is stopped, your bones begin to play 'catch up', and evidence shows that there is little difference in bone density among women aged 75 whether or not they have used HRT. To sustain bone protection, a woman has to begin HRT at menopause and never stop - at which point, the dangers of long-term hormone treatment begin to multiply.