If a nine year old does not have the ability to perform a complete and proper yibum, as is evident from the din that his yevama still requires chalitza (unless he completed his yibum when he became a gadol) then why is he permitted to perform yibum in the first place, since it is not really yibum and thus she is simply an erva to him, since she is his brother’s wife?
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Perhaps one may answer, although a katan is not capable of being koneh the yevama (see tosfos kidushin 68a) however since she is zakuk to him and awaiting his eventual proper yibum (when he becomes a gadol) she does not constitute an ervah and is permitted to him, even while he is yet a katan.