Bava Batra 155 - To Sell Inherited Land One Needs to be Grownup
Since young people do not know the value of land, they can be easily deceived by money offers. The Sages therefore instituted that one can not sell land inherited from a father until one is twenty. In addition, to accomplish a sale one needs to be physically mature. This usually happens at 12 for girls and at 13 for boys, but may come later.
One person in Bnei Brak sold land he inherited from his father and later died. The members of the father's family challenged the sale, saying that the seller had been a minor at the time of this death. Everyone admitted that he was twenty, but the family claimed that he did not have two pubic hairs.
Rabbi Akiva told them, "Firstly, you are not permitted to defile him through an examination. Secondly, it won't help, since signs of maturity change after death."
Art: Cornelis Ketel : Double Portrait of a Brother and Sister