Four years, and both edges catch people
For most of your working life the catch-up contribution is a single figure that applies from the year you turn 50. For four years it is larger. The window covers the calendar years in which you turn 60, 61, 62 and 63, and it closes for good after that.
Both edges are measured by the age you reach during the calendar year, not your age on the day you contribute. They catch people out in opposite directions. Turn 60 in December and the whole of that year qualifies, from January. Turn 64 in December and none of it does, including the eleven months you spent as a 63-year-old.
Nothing carries forward
Each year stands alone. Room you do not use in the year you turn 61 is not available in the year you turn 62, and the window does not extend to compensate. That makes the final year, the one in which you turn 63, different from the others: it is the last chance at the larger figure and there is no later year to move the contribution into.
It also means the window is worth thinking about as a whole rather than one year at a time. Four years of extra room is a meaningfully different number from one year of it, and it is the number the decision actually turns on. Our super catch-up calculator totals what is still ahead of you as well as what applies this year.
SIMPLE plans have their own, smaller window
There is a version of this for SIMPLE plans, and it is much smaller. It comes from a different subsection of the statute, and quoting the workplace-plan figure to someone in a SIMPLE plan overstates their room substantially. Both are real; they are simply not the same number, and nothing about the wrong one looks wrong.
Two things that interact with it
A higher ceiling needs a higher deferral rate to reach it evenly across the year, which is the opposite of what most people assume when they hear a limit went up. If your plan matches each pay period, pacing matters more in these years rather than less. See front-loading 401(k) contributions.
Separately, if your prior-year wages from this employer were above the threshold, your catch-up has to be made as Roth rather than pre-tax, and that applies to the larger window amount exactly as it applies to the ordinary one. If your plan offers no Roth option at all, the consequence is sharper than it sounds: when your plan has no Roth option.