Early in the MUX, Cobra came up with a process by which they'd hoped to give Cobra High Command eternal youth. To test it they captured and subjected several G.I. Joes to the experimental formula, reverting many of the original Joes back to their prime. G.I. Joe escaped, taking a sample of the formula with them. Nonetheless, Cobra started using the process to de-age some of their own veterans, and G.I. Joe replicated the process, doing the same.
Certain unspecified Cobra and Joes were reverted physically back to their prime, from which some aged normally until a few of them were effected again during the Timewarp TP. Others continue to defy aging by continuing to take the formula, regardless of unknown side effects.
This TP is basically an excuse to play the Joes the way they were in the 80s if you like, instead of the grizzled vets the original Joes would be by 2017. Which Joes and Cobras were affected is intentionally left vague so each individual player can decide how they want to play their character. The Timewarp TP provides an additional excuse.