Testamentary capacity, like the capacity to contract, is usually presumed such that it would be up to someone seeking to contest a will on that basis, to prove otherwise based on a balance of probabilities.
However, proving a lack of the legal ability to issue a valid will is not always difficult. For example, the testator may not of been of the age of majority (i.e. a minor) when the will was signed and in a jurisdiction that sets age limits on who can make a will (eg. §7 of the Wills Act, RSBC 1996, Chapter 489).
Other hindrances to testamentary capacity include mental disability if such mental disability is such that the purported testator was, at the time of signing the will, unable to understand what he or she was doing; a will was or what it does. Age does not, without more, cause any presumption of mental disability.
Conversely, certain debilitating physical disabilities can extinguish, temporarily or permanently, testamentary capacity (eg. coma).
The presence of suspicious circumstances may also give rise to questions as to testamentary capacity.
Furthermore, most of the impediments to capacity known to contract law apply as well to wills and affect testamentary capacity, such as undue influence or duress.
REFERENCES: