When you run a large cohort experiment, i would imagine you balance the age profile to population after you do things. If you can get 1000 students in one day but spend three weeks hunting over 60s willing to be injected.. do you refuse to test 1000 students? No. You'd do something akin to Monte-Carlo random sampling into the age bins to get a distribution of experiments to match your needs. Maybe by the time they learned they had a fuckup on their hands, they'd only tested with the bodies available?