Short answer. No. The Code says persons under eighteen cannot make a will, and it allows no exception for illness or urgency. A document signed at seventeen is not cured by the writer later turning eighteen, and the property will pass by operation of law rather than by the paper.
What the law says
Persons of either sex under eighteen years of age cannot make a will.
Civil Code, Article 797 — Minimum Age of Eighteen. Read the full provision →
What the law says
In order to make a will it is essential that the testator be of sound mind at the time of its execution.
Civil Code, Article 798 — Soundness of Mind Required. Read the full provision →
What the law says
nor is the will of an incapable validated by the supervening of capacity
Civil Code, Article 801 — Supervening Incapacity. Read the full provision →
What the law says
The rights to the succession are transmitted from the moment of the death of the decedent.
Civil Code, Article 777 — Succession Opens at the Moment of Death. Read the full provision →
The rule is flat, and it is about the person
Article 797 is one sentence with nothing in it to argue about: Persons of either sex under eighteen years of age cannot make a will. It carries no proviso for a grave illness, no relaxation where death is near, and no allowance for maturity or independence. Because it is a rule of capacity rather than of form, it applies identically to a holographic will written in the testator's own hand and to a notarial will executed before witnesses, and no notary, physician or family agreement can dispense with it. This is not a technicality a court can excuse; it is the condition on which the power to make a will exists at all.
Capacity is measured at the signing, and only then
Article 798 fixes the moment for the companion requirement of mental capacity — In order to make a will it is essential that the testator be of sound mind at the time of its execution. — and the same instant governs age. Article 801 then closes the door on the natural hope that time will fix the problem: supervening incapacity does not invalidate an effective will, nor is the will of an incapable validated by the supervening of capacity. So a paper signed at seventeen does not quietly become a will on the eighteenth birthday; it would have to be made afresh once the capacity to make one exists.
What governs the property instead
Article 774 describes property passing either by will or by operation of law, and Article 777 provides that The rights to the succession are transmitted from the moment of the death of the decedent. With no valid will the second route applies, and the family does not get to choose the outcome. Who takes, and in what proportion, is determined by which relatives actually survive — parents and siblings are the usual candidates — and, where property is held under a marital regime, by that regime being liquidated first so that only the decedent's own share is distributed. Those proportions should be worked out on the real family tree.
If something has already been signed
It cannot take effect as a will, and treating it as one only postpones the disappointment. Say plainly what it is, keep it, and let it stand as a record of what the young person wanted, which relatives sometimes choose to honour voluntarily once the estate is theirs to deal with. Meanwhile the useful work is factual: establish what is genuinely registered in the sibling's name, whether it was bought with someone else's money, and what documents exist. Until the heirs divide, Article 1078 holds the estate in common among them subject to the debts, so nothing has to be settled in a hurry.
Related provisions
- Civil Code, Article 797 — Minimum Age of Eighteen
- Civil Code, Article 798 — Soundness of Mind Required
- Civil Code, Article 801 — Supervening Incapacity
- Civil Code, Article 774 — Succession Defined
- Civil Code, Article 777 — Succession Opens at the Moment of Death
- Civil Code, Article 1078 — Co-Ownership Before Partition