http://lowcarbdiets.about.com/library/blbodyfatcharts.htmAccording to this chart, 18% would still be pretty high for a male body fat percentage, but as long as you're Pre-T, I don't think any of us can rightfully expect you to magically distribute your body fat in a way that suits male levels. Females are prone to having more body fat than men are, so if you start T and you hit the gym, then I'd say your current goal of 15% is more than achievable. My body fat has gone down significantly since I started T because I live a healthy and athletic lifestyle. However, I've heard stories of guys who don't and most of them have put some lovely chub on their abdominal area.
Assuming you have normal female distribution (thighs, hips, butt, and breasts) I'm going to guess that most of your fat is not on your stomach area, but if you don't lose any weight and are already have "a little extra" (and I'm assuming you don't because this chart puts you below the average female range), then T will certainly make that more obvious by moving most that fat to a more centralized area.

I'm currently around 10% body fat though...well, according to my body fat caliper anyway. I don't really know what it was Pre-T, but my body mass was also about 20 pounds less than it is now.