Individuals who are age 50 or over at the end of the calendar year can make annual catch-up contributions. This means you may begin making catch up contributions during the year in which you turn 50 (even while you are still 49). 401(k) and 403(b) plan participants must make catch-up contributions to a retirement plan via elective deferrals. Catch-up contributions must be made before the end of the plan year.
Visit https://www.irs.gov/retirement-plans/plan-participant-employee/retirement-topics-catch-up-contributions to learn more.