Ehm, I personally think that the reason why the 25-45 tend to visit less transgender sites is that they are (statistically) too old and mature to need as much help or be in as much distress as the younger ones, but still not old enough to have a lot of free time to just come hang out and help people around, especially since it's an age range where career, family and children often intersect for very busy days. Plus, half of those people are old enough to not have been in contact with computers in their youth, and maybe the technology became popular when they were very busy with their non-computer jobs and/or family, resulting in them never actually taking the habit of using the Internet at home.
Might I also mention that both the 10-25 (very mildly) and the 65+ (very strongly) age ranges represent baby boom generations? So in the population in general, the 25-45's are a little underrepresented to begin with.
And everyone, please keep in mind that especially in a historical comparison, there is very little correlation between the number of transitioners and the number of transsexuals... There aren't more transsexuals in Thailand, for example; it's just that their culture being what it is, transition paths are more open, so either transsexuals tend to transition more, either non-transsexuals tend to transition more since it's somewhat valued over there. Plus, Asians tend to have it easier, physically, when they transition.