But I agree with Celia, my experience has been that there's a group of girls for who the sexual side seems to be one of the main reasons they do it, and others who it's of secondary or little importance. From what I've seen, age doesn't seem to make a huge difference.
I think the sexual aspects have a couple roots. Most of us began around puberty and at that age anything having to do with girls had sexual overtones.
Plus for many of us, dressing is a chance to express a sexy side of ourselves, which we often feel we can't do so in guy-mode. To give a quick example, back when I was in college and buffed out, I once asked a girlfriend if I could dress sexy for her. She thought about it and suggested I wear a suit.
An acquaintance of mine has an interesting theory about self-as-a-woman fantasies being a coping mechanism for dealing with gender discomfort. (http://members.tgforum.com/jamie/dmt/index.html ) Jamie is living a full-time TG, so her focus is more on TG/TSs, but I think it's still relevant for CDs.
For some girls, I think it's less threatening to think of themselves as "perverts" (i.e. fetistic transvestites) rather than acknowledging that they've got some degree of gender discomfort.
And of course, some (both CD and TS) are just horny.
Darla