I don't remember exactly what age it was at, probably around junior high, but that's when I first started trying on women's clothes (it started with bras and slips I found in the hamper and then graduated to sneaking stuff from my mother's dresser when I was home alone).
Why was I first attracted to putting on women's clothes? Don't know for sure - probably just the sexual excitement a young pubescent encounters when seeing ladies intimate clothing. The JC Penney and Sears catalog intimates sections were like Playboy to me. Putting on women's clothes just took it to the next level for me.
Why do I continue to dress in women's clothes? For me, it's the thrill of looking sexy in sexy clothes. I don't find myself as sexy dressing as a man, but I do as a woman. I also love the way women's clothes fit and feel against my skin.
And yes, there is still a sexual thrill to dressing as a woman - my libido definitely kicks up a notch or two when I'm dressed and seeing myself dressed as a woman is still a sexual turn-on for me.