Blanchard is kind of an idiot. Also, any study of sexuality that doesn't include bisexual is seriously questionable. (Blanchard dumped all the bi people into the "non-homosexual" category, for the record.)
Yes, younger transitioners who are gay exist - I've met a few. It is less common though.
For me personally it was kind of hard to identify with transsexualism when I was growing up 'cause the expectation is straight and very femme. I actually identified as a gay girl before I considered myself trans. A lot of the earlish transitioning gay trans girls have described that problem - it seems to add yet another obstacle on top of the already staggering list. I'm sort of on the line between older and younger transitioners having started HRT at 26.