This is the letter that I wrote to the Watchtower Society requesting my baptism be annulled. I reasoned that my baptism was a void contract as I was a minor and had based my decision on lies contained within the Watchtower, with references to these lies were. The annulment was disallowed on the basis that I had formed other contracts with the Watchtower Society, such as becoming a Pioneer and Bethelite when no longer a minor. The subject matter of the letter was never addressed, nor the actions of the Watchtower Society ever addressed. Therefore I can not say if they will or will not allow your anullment. You need to address it to Bethel rather than the elders, though it may be sent back to the elders in your congregation.
http://jwfacts.com/experiences/grundy.htm
I know of three people that have had their baptisms annulled, however on grounds other than age. Two got rebaptised - they committed offences that they had not known were wrong, so rather than be d/f they were annulled and rebaptised. The third was annulled on the grounds of insanity. After being baptised she continued fornicating, and so the annulment meant she could leave without being d/f.
You do need to be aware that such a request, if not complied with, will result in your being d/f. Even if you are not d/f but annuled you will probably be treated as though disfellowshipped anyway.