Ma and Pa Kent got turned into teenagers and Clark had to sit on them to keep them from doing crazy, irresponsible things like moving to Metropolis.
Superboy 126 was apparently published in 1966. Martha and Jonathan drink some kind of fluid plot-device that turns them physically into adolescents (signified by their use of pre-Depression slang), and apparently also regresses them mentally, because it's heavily implied by the cover that Superboy, an adolescent himself (and absolutely no fun at parties), has to take responsibility and prevent them from doing totally crazy things like deciding they no longer want to live in Smallville. Who would want to move to Metropolis, anyway? It's only the City of the Future. Better to stick around and do vague, nondefined things on your farm.
Much thanks to the Internet, and specifically (postmodern barney).