The attack took place on Wednesday at an old people's home south of the Spanish city of Avila.
The cause of the attack was not yet known, sources close to the investigation told Spanish news agency Efe.
Those sources added there was no known history of bad blood between the two men.
But a different source said the murder suspect was suffering Alzheimers and had previously complained his neighbour talked too much.
The presiding magistrate in the case has now asked for a report on the psychological state of the alleged murderer.
The attack took place in the town of Buenaventura in Spain's Toledo province.
In 2009, in the same municipality, an 80-year-old man shot his 75-year-old brother with a rifle.