"It may be that our role on this planet is not to worship God - but to create him."
-Arthur C. Clarke

Granted the inexplicable nature of this technology that we don't entirely understand (having possibly been co-opted from some manner of alien relics or artifacts), it's possibly the machines simply reject anyone past a certain age, for reasons not yet understood. That would be the hand-wavey method of working it in, but it'll buy you time to think of a clever plot twist to work in later.