While the majority of breast cancer is hormone positive as you rightly say, @AT404, that does not necessarily mean that hormones are the cause, or should I say the only cause of this type of breast cancer. Cancer is a complicated disease that they have not been able to really find the exact cause of. I suspect that it is not just one cause but in fact many and possibly a little bit different for each of us.
Maybe it it is just more likely that older women will have whatever starts this terrible process occur and then any hormones (and as we know estrogen is still produced by women, even after menopause) are used by the cancer to grow.
As @melclarity says, hormone positive breast cancer is certainly occurring in women before menopause in growing numbers too. I was only 47 at diagnosis for example. I hope one day soon that they can give us more answers.
Maybe your doctors can answer this with more clarity.