Deep down, in some part of our aging brain, the old person we're rapidly becoming shakes his head and wonders, "What is this world coming to when teen-agers feel they need a spa?" Of course, in some other part not quite so deep, we're humming "Thank Heaven for Little Girls" and thinking this is just a fine idea. Apparently, so do the youngsters themselves. This spacious spa in a Plano shopping center offers a full salon, makeovers, makeup instruction and massages, all geared to the target audience of Seventeen magazine, which has a licensing agreement for the name. (The Plano location is the first of 36 planned.) Why would a young person need a facial or massage? Acne, for one, we suppose, but teens also face an inordinate amount of stress, hence the popularity of massages. The spa also offers services to boys, though, by gum, in our day no self-respecting boy...oh, never mind. We're old. If you're not, and you have a sense of style or need help in how to apply glitter makeup, this might be the place for you. They also offer nifty gift cards.