Bava Batra 156A

ולטעמיך, זבין שוי חמשא ⁠– בשיתא, הכי נמי דזביניה זביני?!

And according to your reasoning, that the money he receives in exchange for the property is a reason one could consider his sale valid, if he sold property worth 5 dinars for 6 dinars, would his sale also be a valid sale?

אלא קים להו לרבנן דינוקא מקרבא דעתיה גבי זוזי; ואי אמרת זביניה זביני, זמנין דמקרקשי ליה זוזי, אזיל מזבין לכולהו נכסי דאבוה. אבל גבי מתנה, אי לאו דהוה ליה הנאה מיניה ⁠– לא הוה יהיב ליה מתנה; אמרו רבנן: תיהוי מתנתו מתנה, דלעבידו להו מילי.

Rather, the rabbis maintain that a child’s inclination is to be attracted to money. And if you say that his sale is a valid sale, there may be times that there are potential buyers who rattle the dinars before him in order to tempt him to sell, and he will go and sell all of his father’s property. That is why the rabbis ruled that all of his sales are not valid. But with regard to a gift, if he did not derive benefit from the recipient, he would not give him a gift. The rabbis therefore said: Let the gift of an orphan be a valid gift, so that people will perform beneficial matters for the orphans, as the orphan can reciprocate by giving gifts.

אמר רב נחמן אמר שמואל: בודקין לקדושין, לגרושין, ולחליצה, ולמיאונין. ולמכור בנכסי אביו ⁠– עד שיהא בן עשרים.

§ Rav Naḥman says that Shmuel says: Children who have reached the age of majority, i.e., a boy who is 13 years old and a girl who is 12 years old, are examined for signs indicating puberty if it is necessary to determine their adulthood for the purpose of betrothal, for the purpose of divorce, for the purpose of ḥalitza, and for the purpose of stating a girl’s refusal to remain married. But in order to sell from the property that one inherited from his father, the seller must be older, and one cannot sell this property until the seller is 20 years old.