What is the effect of senility on a will's validity?

That depends on when the senility occurred. If the deceased was competent at the time the Will was written and signed, later senility won’t have any effect on the Will’s validity at all. If the deceased was senile at the time the Will was made, the Will might be invalid, but this isn’t a simple question.

Don't think that just because the deceased wasn't as sharp as she used to be, or that because he was old and forgetful, that this person was necessarily legally incompetent to make a Will. The person seeking to have the Will accepted for probate generally has to establish that the deceased was of sound mind, and the witnesses to the Will signing usually say the deceased was of sound mind. Then the burden shifts to the person challenging the Will to prove it should not be admitted to probate because the decedent was not competent.

It can be very difficult, and costly to prove that the deceased was mentally incompetent. The questions are: Did the person making the Will (1) Know what a will is and that he or she was making one;

(2) Understand the relationship between the decedent and the beneficiaries of the Will;

(3) Understand what he or she owned; and

(4) Have the ability to decide who should get what?

This is not a very high standard, so it is very difficult to prove that someone was incapable of making a Will.

The question of senility will only come up if someone challenges the Will. If the affidavits of the people witnessing the signing of the Will say the person knew what he or she was doing, the probate court will usually accept that as true if there is no challenge.