Maybe it's just that I work in a semi-business casual environment, where the women wear anything from twin sets and pants to full out suits, but I never really understood why some petite women have a preference for the junior section. And this goes for some women who are in their mid 30s. The most frequent thing I hear is that juniors fit better. Do they? Juniors clothing may be cut smaller, but they don't really fit a petite frame all that well. It's still cut for a taller figure, for one. Also, it's got a younger look to it. The one thing petite women don't want. Many of us do look younger than our age, and when you look young, especially when you are a shorter person who looks young, you may very well be mistaken for the intern rather than the someone who is in charge.
If you work in downtown type business environment, then a suit is very important, even if your office is business casual...you'll still need a suit for more formal occasions, such as when you're meeting with clients. Personally, I'd suggest that you spend more than "normal" on this, and definitely not in the juniors department. I have found that most of their suits are not that great quality and aren't cut properly. Ideally, one should go petite, but I realize that not every place makes them. Even better, you can go custom, in places such as AprilMarin (but you'll need to know your measurements inside out if you want to get a perfect-perfect fit.) A grown-up should get a grown-up suit. After all, you should be dressing for the job you WANT, not the one you have. You're not doing that if your suit is from the juniors' department.
