Yevamos13: 12
יָבָם קָטָן שֶׁבָּא עַל יְבָמָה קְטַנָּה — יִגְדְּלוּ זֶה עִם זֶה. בָּא עַל יְבָמָה גְדוֹלָה — תְּגַדְּלֶנּוּ.  הַיְבָמָה שֶׁאָמְרָה בְּתוֹךְ שְׁלֹשִׁים יוֹם ,,לֹא נִבְעַלְתִּי” — כּוֹפִין אוֹתוֹ שֶׁיַּחֲלוֹץ לָהּ. לְאַחַר שְׁלֹשִׁים יוֹם — מְבַקְשִׁים הֵימֶנּוּ שְׁיַּחֲלוֹץ לָהּ. וּבִזְמַן שֶׁהוּא מוֹדֶה — אֲפִלּוּ לְאַחַר שְׁנֵים עָשָׂר חֹדֶשׁ כּוֹפִין אוֹתוֹ שֶׁיַּחֲלוֹץ לָהּ.
[If] a minor yavam cohabited with a minor yevamah, they must wait for each other to grow up. [If] he cohabited with an adult yevamah, she must wait for him to grow up.  [If] a yevamah stated within thirty days, “He did not cohabit with me,” we coerce him to perform chalitzah with her; [if she stated this] after thirty days, we request of him that he perform chalitzah with her. However, if he admits [to it], then even after twelve months we coerce him to perform chalitzah with her.