Contract的首要条件就是--Legally competent parties。
Parties entering into a contract must have legal capacity to do so.
Almost anyone is capable, with a few exceptions：
A person must be at least 18 years of age, unless married, in the military or emancipated.
A minor is not capable of appointing an agent, or entering into an agency
agreement with a broker to buy or sell a real estate!
but Both minors and incompetents may acquire title to real property
by gift or inheritance.