Terri,
The mechanism of action of Tamoxifen is the same in both pre- and post-menopausal women: it is a weaker form of estrogen than the body's own that competes for the estrogen receptor on the cell. One reason that pre-menopausal ER+ women are rx chemotherapy is to shut down the ovaries. This can be achieved hormonally or via their removal, however. When a woman's ovaries no longer produce estrogen, the body still requires it and a single enzyme converts aromatase into estrogen. Aromatase inhibitors prevent this action, and work via strictly estrogen deprivation - there is no circulating estrogen to attach to the receptor. AI's are thought to be more effective in treating Her2+ bc.
I don't know why ER+ tumors are more common in postmenopausal women, not do I recall reading a paper that addresses this. It seems counterintuitive, but there it is. I think if science could find an answer, we would be further along the road to prevention.
Here is a link to a terrific site with excellent information on all aspects of bc:
http://home.earthlink.net/~ckane/brca.htm. There are lots of articles on hormonal therapy cited, one of them may even have those stats you are seeking.
Best of luck to your wife (and you!) with her treatment plan,
Hopeful