A person under age cannot make a will.
An insane person cannot make a will except in a lucid interval.
Very clearly one who is insane cannot give his property by will, although if he has lucid intervals he may make a will during such interval.
A party must understand the nature of his act in making a will.
A person who makes a will must be strong enough mentally to understand the nature of his act. He need not know enough to perform his ordinary business affairs but he must know what he is doing. Thus, an old man who has become too feeble minded to perform ordinary business affairs may yet know enough to make a will, provided he understands that he is disposing of his property by will.