Suppose you have a will that is 20 years old. Maybe it leaves practically everything to your ex-wife and nothing to your new wife. Would such a will still be valid?
Yes, it can be sometimes. However, the age of the will may raise some issues.
Many life changes in the meantime
If you have had many life changes since you made your will, someone could make an argument that the will no longer represents your wishes. For example, suppose you got divorced and then remarried. You changed your life insurance and retirement plan beneficiaries to the new spouse while the will provides for your ex-spouse with no mention of the new spouse.
However, maybe you wanted it that way. Perhaps you remained on good terms with your ex, and your new spouse has independent means. Still, to cover your bases, it is a good idea to revisit your will and other estate planning documents with a lawyer every few years to minimize the chances that someone could successfully challenge them. In fact, Washington state law even says that if you divorce after making a will, the presumption is that you do not want your ex to inherit anything unless your will says otherwise.
It also helps to tell any potential beneficiaries (and those who might be surprised to not be in the will or to have a reduced role) about your plans. That way, there are no surprises when the time comes.
The appearance of a newer will
If you have a new(er) will, it should be valid over the old will in most circumstances. Some of the exceptions might be if you did not sign the newer will properly or did not follow state requirements in other ways. Many new wills have a clause that says this new will invalidates older wills, but even without such a clause, the new will should still be valid if you did everything in accordance to the law.