Fantastic Four, Vol. 1 #214 (1979)

...And Then There Was--One!
Johnny is left, with the other three in stasis. He tries to get help from Tony Stark, but Stark is working with radioactive waste for S.H.I.E.L.D. and cannot come out without lengthy decontamination. Johnny returns to the Baxter Building, only to face off against Skrull X, the robot Skrull that first attacked them way back when this saga started in issue 204. Johnny wins the battle, and takes the Skrull aging ray from this Skrull. Waking Reed, they modify the device to reverse the process, and Johnny's teammates come back younger and more vital than ever.
Release Date: 1979