Your TEFL certificate / qualification has no expiration and will remain valid indefinitely. However, the industry does change over time as do expectations from schools and employers which is why the course has evolved over the years.
We currently offer a 168-hour Level 5 course. We upgraded to this course from the 120-hour course for this very reason, to offer the most current version of the TEFL qualification available, but students who previously earned the 120-hour certificate through our course are still using it to successfully apply for teaching jobs worldwide as it has remained valid.
Some schools do also give preference to teachers who are more recently qualified as the methodology learned is up to date and their knowledge is fresh, so it is wise to do some research into the schools in the country or region you are hoping to teach in to determine what their specific requirements are.