As an *ahem* older transitioner, I have noticed a wide spread in age for MtFs, while most FtMs are a bit younger (mid-40s and below). I think this is just because of historical reasons.
Surgery was first available for MtFs and so has developed further than corrective surgery for FtMs. I think the idea of MtFs has been around longer than FtMs. (I had never even thought there were FtMs until about 20 years ago, but then I
can be woefully clueless.

)
If we ever get to a steady state of full social acceptance and well-developed corrective surgery, I think the balance will be about even between MtFs and FtMs, the backlog of older transitioners will be cleared out, and "older" transitioners will be those who are doing it after age 18. But we're a long way from that.
- Kate