An extra allowance, not a bigger one
Every workplace retirement plan caps what you may contribute from your own pay each year. From the calendar year you turn 50, you may contribute an additional amount on top of that cap. It is a second allowance sitting above the first, not a larger version of it, and the distinction matters because your plan will stop you at the two added together.
Age is measured by the year you reach, not the day you contribute. Turn 50 in December and the whole of that year counts, from January.
The figure depends on your plan and your age
There is no single catch-up number. A 401(k), 403(b) or governmental 457(b) uses one figure; a SIMPLE plan uses a smaller one from a different part of the statute. Quoting the workplace-plan figure to someone in a SIMPLE plan overstates their room by thousands, and nothing about the wrong number looks wrong.
Age changes it again. For four years, the calendar years you turn 60 through 63, the catch-up is larger, and then it reverts. That window and what it is worth across the whole span is covered in the 60 to 63 contribution window, and our super catch-up calculator works out the limit that applies to you now.
Two things that interact with it
If your prior-year wages from this employer were above the threshold, the catch-up has to be made as Roth rather than pre-tax. See the SECURE 2.0 Roth catch-up rule.
And if your plan matches each pay period, a higher ceiling needs a higher deferral rate to reach it evenly across the year. Reaching any limit early stops the match for the rest of the year unless the plan trues up, which is the subject of front-loading 401(k) contributions.