The bible defines work as a gift from God. It doesn't talk about retirement. Where did this concept come from?
I have heard many times that the Bible doesn't speak of retirement.
But there's an interesting passage about the Levites in Numbers: "But at the age of 50, they must retire from their regular service and work no longer." -- Numbers 8:25
I've read that this was because 50 was seen as an age where a person's abilities started to diminish, and the temple duties included things like heavy lifting and good eyesight (inspecting skin diseases, etc.) Without the availability of glasses, many at that age might be less effective at the work that was required.
But the article also notes that this did not mean the person was to stop all work and become useless. Numbers 8:26 says that they were still to "assist their brothers in the tent of meeting in carrying out their duties."
I've personally never seen retirement as, "Stop all work and become an unproductive blob."
Rather, I always saw it as a shift -- a time when someone might have the privilege of choosing what kind of work to take on and how much, instead of feeling it was an obligation -- especially for those who don't have the blessing of a job they enjoy.
Everyone I know who retired never stopped working -- they took on lower-stress part-time jobs and/or increased their time volunteering/doing ministry, which to them, was "for fun", and didn't feel like actual "work" -- even though they were twice as busy as when they had a regular job.