I flew with pilots that turned 60 and moved to the flight engineer seat, and when the age moved to 65, they returned to the left seat until retiring back to the FE seat again. In fact, I represented a grievance for an operator who allowed the captain to keep his seniority when moving to the FE seat, but had no language to address a second move (in which the decision was to move the re-retiree to the bottom of the FE list).

When a contract is amid ships and the unprecedented occurs due to a regulatory change, in limited case it might be settled out that way, or by an MOU. Without a FE seat to go to, one has to look closely at any language surrounding retirement; few contracts will have addressed returning from retirement, and none, from age-based retirement.

(I realize that one who moves to the FE seat, in the former case, has not "left 121," but the shift is notable, none the less. What of the captain who retired at 65 to the training department, but who wishes to return to the line for another year?)