You can sue anybody, winning is another matter. If the GA insurance Commissioner's office said you can, then probably so. Best bet would be to see an attorney to explore your options.
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It depends on why the policy was supposed to be transferred to you upon you turning 18. Was this provision in the policy or did the policy owner make this promise to you? You cannot sue the insurance company if the policy owner never kept his/her word to transfer the ownership to you. If, however, the insurance company was obligated under the terms of the contract to initiate the transfer to you when you turn 18, and it failed to do so, then you may have a valid claim for breach of contract and possible bad faith. Try writing to United's legal department demanding the policy.