Short answer. Yes, that situation is exactly what the Rules contemplate. Rule 92, Section 2 counts among incompetents persons who, by reason of age, disease, weak mind and similar causes, cannot without outside aid take care of themselves and manage their property. Guardianship of incompetent adults remains governed by the Rules of Court.
What the law says
persons not being of unsound mind, but by reason of age, disease, weak mind, and other similar causes, cannot, without outside aid, take care of themselves and manage their property, becoming thereby an easy prey for deceit and exploitation
Rule 92, Section 2 — Meaning of word "incompetent.". Read the full provision →
Unsound mind is not required
The definition's last limb is the one that fits an ailing parent. It covers persons not being of unsound mind, but by reason of age, disease, weak mind, and other similar causes, cannot, without outside aid, take care of themselves and manage their property, becoming thereby an easy prey for deceit and exploitation. Notice what that does not demand: no finding of insanity, no diagnosis of a mental disorder. A parent who is lucid in conversation may still be an incompetent in this legal sense if age and illness have left them unable to handle money and property without help.
The test is functional, and the concern is exploitation
What the court asks is not what condition the person has but what the condition has done: can they, without outside aid, take care of themselves and manage their property? The definition names its own worry in the closing words — becoming an easy prey for deceit and exploitation. That is the scenario families usually bring to a lawyer: a parent signing documents they no longer understand, money disappearing to persuasive strangers or persuasive relatives. Guardianship exists so that a court-appointed and court-supervised person, rather than whoever gets to the parent first, controls those decisions.
Adults under the Rules of Court, minors under the 2003 Rule
One point of housekeeping about which rules apply. Rules 92 to 97 of the Rules of Court no longer govern guardianship of minors: A.M. No. 03-02-05-SC, the Rule on Guardianship of Minors, effective May 1, 2003, took over that subject, while providing that guardianship of incompetents who are not minors continues under the Rules of Court. For an elderly parent, then, this definition and the procedure built around it are the governing law. The same definition also lists other classes — among them persons under civil interdiction and those of unsound mind even with lucid intervals.
What a family should be ready to show
Because the test is functional, the useful evidence is concrete: medical records describing the condition, and specific incidents showing its effect on the parent's affairs — unpaid obligations, unexplained withdrawals, transactions they cannot recall or explain. Remember, too, what guardianship costs the parent: it takes away control of their own person or property, so a court will want to be satisfied the inability is real, not merely that the family disapproves of their spending. If the parent can still manage with support, talk to a lawyer about whether guardianship is the proportionate tool before filing anything.