Instead of approaching her with "please don't", why not approach her with "please do". Tell her that you feel that you missed out on a lot of things because you were baptized a her age and that you don't want the same to happen to her. Ask her what she's interested in doing...extra-curricular activities at school, advanced placement classes, clubs, organizations, etc. She should also be thinking about college applications and all that entails such as volunteer work, essays, scholarships, etc.
There are so many healthy, helpful, and valuable activities, other than field service and preparation for baptism, that she could be doing. Direct and encourage her in those things, and she won't need baptism to establish credibility or gain self-esteem.