We know that Jewish law recognizes a child as reaching maturity when he becomes physically mature. Nevertheless, with regard to certain halakhot the law may require other types of maturity.
Our Gemara presents a case where a child has received an inheritance from his father. How old must he be before he is allowed to sell that property? Rava quotes Rav Naḥman as requiring him to be 18 years old; Rav Huna bar Ḥinnana quotes Rav Naḥman as requiring him to have reached 20 years.
Some of the Ge’onim understand that this question does not relate specifically to inheritance, but to any business transaction – the Gemara is simply discussing the most ordinary case where a young person will have property to deal with. The concern is that a young child will not possess the business acumen to buy and sell, so Jewish law will consider him a minor until he becomes older.
Most of the commentaries (e.g., the Ri”f, Rambam, Meiri and others) apply this only to property received as an inheritance from his father, while the Ritva applies it to any situation of inheritance, even from his mother or his siblings. The reason to limit it to apply only in the case of a father’s inheritance is that this is a rabbinic enactment to protect the value of the estate, and such enactments are limited only to ordinary cases. It is common for a child to inherit his father’s estate – any other inheritance would be unusual.
Although the continuation of the Gemara seems to imply that the ages of 18 or 20 are dependent on physical maturity, it is clear that the reasoning behind this law is our concern that a young person makes well thought-out financial decisions. In fact, Rabbeinu Tam and the Ri”d both rule that a child of 13 years old who is known to understand the markets and is savvy in his business sense would be permitted to buy and sell this property.