intestable

from The Century Dictionary.

• Legally unqualified or disqualified to make a will: as, an idiot or.a lunatic is intestable.

from the GNU version of the Collaborative International Dictionary of English.

• adjective (Law) Not capable of making a will; not legally qualified or competent to make a testament.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective law Not legally permitted to make a will, as by reason of being under the age of majority or mentally incompetent.