I have a friend who tried to have her baptism "annulled," as she was only 12 at the time, and under pressure from her parents. She was 19 when she left the Borg, and when they eventually went to df her, she claimed that her baptism was invalid as it was under duress and she didn't understand what she was agreeing to.
However, the elders were having none of that--wanted to make an example of her. She was df'd anyway.
I wonder if she'd have had a legal case--clearly she was under the age of consent at the time of the baptism.
Jankyn