There is no evidence that "late pregnancies" naturally or artificially conceived, have any impact on overall breast cancer rates. There was a small study some years ago that thought it found a small connection between fertility treatments and ovarian cancer. However, further research confirmed that the elevated cancer rate was associated with infertility, regardless of whether the women in question had ever undergone fertility treatments.
I think the connection may be through the manipulation of hormones, similiar to the link found in HRT/ birth control.
that would be the possible link.