This may sound radical, but trust me. I'm sure this will work.
First, implant red crystals in the palm of everyone's left hand. Trigger the crystal to blink whenever the person it's attached to turns 30.
Next, clad all 30-year-olds in white robes and grotesque masks.
Gather everyone in a circle inside a large auditorium.
Begin Carrousel: Have everyone in the circle explode in midair.
And voila! No need for Social Security benefits.
Oh, it may work better if you come up with some cover story, like the land outside your domed city (e.g., Canada) is uninhabitable, so people won't try to escape.