Commentary for Kiddushin 129:4

The Talmud rejects the difficulty. In the first half of the statement, the youngest daughter of the first group is mentioned. Any girl older than her is part of the doubt, for all of these girls are older than those in the second set. However, when it comes to the second group, the mishnah mentions only the oldest. Had there been a doubt about any daughter younger than her, then the mishnah should have mentioned her.