The legal requirements for drafting a valid will aren't nearly as complicated as many people fear. There are just a few simple rules; follow them and you'll leave a will that you can rely on to make sure your wishes will be carried out.
To make a will, you must either be:
18 years of age or older, or
living in one of the few states that permit younger persons to make a will if they're married, in the military, or otherwise considered "emancipated."
You must be of "sound mind" to make a valid will. It's not a rigorous requirement. The standard interpretations require that you:
know what a will is and that you're making one
understand the relationship between yourself and those persons whom you would normally provide for, such as a spouse or children
understand what you own, and
be able to decide how to distribute your property.
In reality, a person must be pretty far gone before a court will rule that she lacked the capacity to make a valid will. For example, forgetfulness or even the inability to recognize friends doesn't, by itself, establish incapacity. Also, it's important to remember that in the vast majority of cases, there's no need to prove mental state to a court. It's presumed that the will writer was of sound mind, and the issue will never arise unless someone challenges this in a court proceeding -- which is very rare.
A will can also be declared invalid if a court determines that it was procured by "fraud" or "undue influence." This usually involves some evildoer manipulating a person of unsound mind to leave all, or most, of his property to the manipulator. Will contests based on these grounds are also quite rare.
If you suspect that someone might challenge your will on the basis of your mental competence or fraud, be sure to see a lawyer. For example, if you plan to leave the bulk of your property to someone you know is disliked and mistrusted by some close family members, work with a lawyer to minimize the possibility of a lawsuit, and maximize the chances that your wishes will prevail if there is one.