Bava Batra 155B

היינו דקאתו ואמרו ליה: מהו לבודקו. אלא אי אמרת מבן עשרים, כי בדקו ליה מאי הוי? והא תנן: בן עשרים שלא הביא שתי שערות ⁠– יביאו ראיה שהוא בן עשרים; והוא הסריס, לא חולץ ולא מיבם!

this is the reason that they came and said to R' Akiva: What is the halakha? Is it permitted to examine the corpse? But if you say that he can sell the property from the time he is 20 years old, even if they examine him, what of it? But didn’t we learn in a Mishnah (Nidda 47b): With regard to a 20-year-old man who did not develop two pubic hairs, proof must be brought that he is 20 years old, and then he is no longer considered a minor. And he is the sexually underdeveloped man, who is excluded from the mitzva of levirate marriage. Therefore, if his married brother dies childless, he neither performs ḥalitza nor enters into levirate marriage with his widow. Since a 20-year-old is considered an adult even if he has not developed signs of puberty, there is no reason to examine the body.

לאו איתמר עלה, אמר רבי שמואל בר רב יצחק אמר רב: והוא שנולדו בו סימני סריס? אמר רבא: דיקא נמי, דקתני: והוא הסריס; שמע מינה.

The Talmud replies: Wasn’t it stated with regard to that Mishnah: R' Shmuel bar Rav Yitzḥak says that Rav says: And is this the halakha only where he developed the signs of a sexually underdeveloped man? Otherwise, a 20-year-old who has not developed two pubic hairs is still considered a minor. The examination of the deceased could therefore be effective to see if he has other signs of being sexually underdeveloped. Rava said: The language of the Mishnah is also precise, as it teaches: And he is the sexually underdeveloped man. The usage of the definitive article indicates that the Mishnah is referring to one who is clearly a sexually underdeveloped man. The Talmud affirms: One can conclude from the Mishnah that the Mishnah is speaking of one who developed the signs of a sexually underdeveloped man.