Fantastic Four (1961) 245-A

"Childhood's End"

The Invisible Girl discovers that the rest of the team have been defeated in the Baxter Building by a bearded, long haired stranger seeking an answer to an unknown question. It is soon discovered that the man is actually Susan's son, Franklin; whose latent mutant abilities have surfaced and unwillingly caused him to age into full adulthood. Before reverting back to childhood, Franklin uses his power to revert Ben into his familiar rocky form making it possible again to one day be cured.