Short answer. It normally keeps running. Article 606 of the Civil Code says a usufruct granted for the time before a third person attains a certain age subsists for the number of years specified even if that person dies first — unless it was expressly granted only in consideration of his existence.
What the law says
A usufruct granted for the time that may elapse before a third person attains a certain age, shall subsist for the number of years specified, even if the third person should die before the period expires, unless such usufruct has been expressly granted only in consideration of the existence of such person.
Civil Code, Article 606 — Usufruct Until a Third Person Comes of Age. Read the full provision →
The child's age is a way of measuring time
The provision reads: A usufruct granted for the time that may elapse before a third person attains a certain age, shall subsist for the number of years specified, even if the third person should die before the period expires, unless such usufruct has been expressly granted only in consideration of the existence of such person. The law treats a reference to a child's coming of age as a clock rather than as a condition. The grantor could have said "for twelve years"; saying "until my grandson turns eighteen" is read as the same thing. So the death of that child, sad as it is, does not by itself end the usufructuary's right.
The exception, and how narrow it is
There is a way out, but the statute guards it with the word expressly. The usufruct ends early only where it was granted only in consideration of the existence of such person — that is, where the instrument itself shows the grant was made because of the child and for the child's sake, not merely measured by his age. A general recital that the arrangement was "for the family" or "for the benefit of the children" is unlikely to be enough on its own. The intention has to be visible in the words, because the article's default is continuation and the burden of displacing a default sits on whoever wants it displaced.
Why the rule works this way
Usufruct separates the enjoyment of property from its ownership for a defined stretch of time, and both sides plan around that stretch. The usufructuary may have taken on repairs, planted, leased, or built a livelihood on the expectation of a fixed number of years; the naked owner has budgeted for the property returning on a known date. Letting the arrangement collapse on an event nobody can control would make every such grant unstable. Reading the age reference as a term, rather than as a condition, gives both parties a date they can rely on and one that can be worked out on paper from the beginning.
What still ends a usufruct, and what to check
This article settles one question only. A usufruct can still end for its own reasons — the death of the usufructuary himself, expiry of the term, merger of the usufruct and ownership in one person, renunciation, total loss of the property, or the fulfilment of a resolutory condition written into the grant. The usufructuary's obligations also continue meanwhile: to preserve the form and substance of the property, to make ordinary repairs, and to return it at the end. So the first step is to read the deed or the will that created the right, word for word, and work out the exact number of years the age reference produces. Where the wording is doubtful, have a lawyer look at it before anyone gives up possession or demands it back.