PRETTIER TEENS: As cosmetic surgery's popularity grows (A1), more teen-agers are seeking it too. But cosmetic surgery is not always appropriate, and now there are some strict guidelines for teens, says Dr. John Strong of Santa Ana, head of the state plastic surgeons' group. . . . "Parents sometimes live vicariously through their children and want them perfect," he says. Another problem: Young people are not mature enough to know if cosmetic surgery is best for them.