All your estate planning work might come to naught if your will is invalidated during the probate process. Working with a professional experienced in all aspects of the estate planning and probate litigation process helps ensure your wishes are honored and your heirs are protected in the manner you wanted them to be. A first step in that process is ensuring your will is legally valid.
Most states require that a legally valid will be written by or for a person with testamentary capacity. This means that you are sound of mind -- mentally and emotionally able to make the declarations and decisions captured in the will. You should be able to understand the fullness of your estate and the fact that you are disposing of assets within the will.
It's not enough to be sound of mind, though. You also have to be of legal age. In most locations, you're legally able to sign a will if you are at least 18, are a member in the United States armed forces or are married or have been married according to the laws of the state. Each jurisdiction is slightly different, making it important to seek legal knowledge about age requirements if you are beginning the estate process very young.
Finally, for a will to be valid, it has to be signed voluntarily by a person with intent. The person cannot be tricked into signing a will or signing a will while experiencing undue influence. If you can prove a will was signed through deceit or that someone was forced to sign a will, it might be thrown out during probate processes.
Source: FindLaw, "What Us a 'Valid Will'?," accessed Dec. 23, 2015