Responsa for Yevamot 67:12

מפרישים אותן: והא אין אשה מתעברת בביאה ראשונה אמר ר"נ אמר רבה בר אבוה שבעלו ושנו ואלא הא דתנא ר' חייא הרי כאן ט"ז חטאות תלתין ותרתין הויין

and one agent met the other;<span class="x" onmousemove="('comment',' So that all prohibitions took effect at the very same moment. ');">³²</span> but how could such [simultaneity] occur with menstruation?<span class="x" onmousemove="('comment',' Which would naturally occur either before, and thus prevent the other three prohibitions from coming into force; or after, and thus be prevented itself from coming Into force. ');">³³</span> - R. Amram in the name of Rab replied: When the women's menstrual discharge continued from [the men's] thirteenth, until after their thirteenth [birthday], when these become subject to legal punishments; and from their own twelfth, until after their twelfth [birthday], when they themselves become subject to punishments.<span class="x" onmousemove="('comment',' A male becomes legally liable to punishments on the termination of his thirteenth, and a female on that of her twelfth year of age. If the respective agents of the two parties who were of the same age to a day. met sometime prior to the conclusion of the last day of the year (twelfth of the females and thirteenth of the males), and arranged for the betrothals to take effect on the following day when both parties become 'of age' (as otherwise the betrothals would not be valid) the betrothals and the prohibitions simultaneously come into force. ');">³⁴</span>

Teshuvot Maharam