Why only younger? All that temporal energy has to go somewhere! Long time ago, DC did a wacky Young Justice-based event where all the adult heroes became kids. And the kid heroes became adults. It gave the writers (mainly Peter David) a chance for cracky fun with the JLA, JSA, etc, but also let them explore what sorts of adults the teen heroes might be.

Never mind that nothing is going to upset a teen more than being 55!

Frankly, I would love a chance to have Kirk in his prime for just a few days.