"Mercury Age?"
Are you guys kidding me? That'll never catch on.
Giving an age a new name is like trying to give yourself a nickname.
So what state are people in when they're in that dimension? They are not flesh and blood. Yet they experience and remember; and if they are changing (they must be), why do they not age? If Mon-El emerges from the Zone "nuts" and "angry", if his brain (or perhaps his mind) can suffer adverse effects, why can't his body? To change is to age. Or, put another way, to go from one mental state to another means you have been subject to Time, and that means you have aged.
Superhero comics work according to a presumption, one that is so implicit that it doesn't need to be explicitly stated: the mind is different from the body.
That's why people in comics universes can survive being turned into "solid sound" or have their minds transferred to Living Colossuses or Nomans. Yeah, Mordru might have just transformed Cosmic Boy into a statue made of glass, but he'll be okay, because his mind is still in there somewhere.
(In a superhero universe, the division between the "soul" and the "mind" are a little less clear than the division between mind and body. It's unclear, for instance, whether it is Doctor Strange's soul, or his mind, or both, that detaches from his body when he astrally projects.)
Thus, going by this piece of "superhero metaphysics," even if in the PZ is a state of constant stasis that traps the Zoners PHYSICALLY, the MINDS of the Zoners can still be up and about, just like Cosmic Boy can survive being transformed into a glass statue. This is supported in the many schemes by PZ-ers to interact with the "real" world via mental powers, or the fact everyone in the Zone can communicate via telepathy.