Short answer. Usually not. A usufruct granted until someone reaches a certain age normally runs for the full number of years even if that person dies first. It ends early only if the grant was expressly made in consideration of that person's existence, in which case their death does end it immediately, before the specified age is reached.
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 default rule: the term survives the person
When a usufruct is set up to last until a third person reaches a certain age, the law's default assumption is that what was granted is a fixed term — a set number of years calculated from the age gap — and not a right tied to that person staying alive. So if the usufruct was meant to run, say, until a named child turns eighteen, and that child dies at ten, the usufruct still continues for the remaining eight years under the default rule. The law treats the person's age as a way of measuring the length of the usufruct, not as the thing the usufruct depends on.
The exception: when it was tied to the person's life
That default gives way when the usufruct was expressly granted only in consideration of the existence of the third person — meaning the grantor made clear that the point of the arrangement was tied to that specific person being alive, not just to a stretch of time measured by their age. In that situation, the person's death ends the usufruct immediately, regardless of how many years remained before they would have reached the specified age. The key word is expressly: this exception does not apply unless the grant itself shows that intent.
Why the distinction matters
Whether a usufruct falls under the general rule or the exception can change who benefits from years the third person never lived to see, and whether the usufructuary's right ends abruptly or runs its full course. Because the default favors continuation of the fixed term, anyone who actually wants a usufruct to end at the third person's death — rather than continue for the remaining calculated years — needs to say so clearly in the instrument creating it. Silence or ambiguity falls back on the general rule that the usufruct subsists for the number of years specified.