I mean literally our breast tissue forms at a later age for many of us transwomen - ergo less time for exposure to risk factors.

or maybe not?

Anyone can get breast cancer male or female. Richard Roundtree who starred as John Shaft in the 1971 film "Shaft" found he had breast cancer. It's probably more common in cis females than trans women as breast tissue over a period of years becomes a repository for ingested carcinogens from alcohol and tobacco use and environmental contaminants such as aluminum chlorhydroxide used in antiperspirants among other things. Their breasts are often encased in foam rubber padded bras for extended periods and the list of potential cancer causing agents goes on.

Also, Dr. Oz had a big shot oncologist on his show a couple of months ago warning about girls who keep their cell phones in their bras.  They are seeing really young woman develop problems from this practice.  Evidently a lot of girls/young woman do it.  According to Cancer.org it hasn't been either proven or unproven yet because it just hasn't been studied.

I've actually asked several healthcare professionals about this, since I'm coming up on the age they used to recommend getting a baseline mammogram. The consensus seems to be more or less in line with my own assumption - our overall risk is higher than a cis guy's (which is, as people have said, not zero), but lower than a cis woman's of the same age. That's disregarding familial and environmental factors for a given trans woman.

Some breast cancers *are* encouraged by estrogen bathing the tissues, so presumably at a minimum the risk is equivalent to the breasts' age if not the woman's (=on HRT for 10 years, roughly the same as a 22-year-old cis woman, etc.).