This is a fun but disjointed and at times directionless little trade collecting what may be one of the most inconsequential crossovers in the history of DC - a fluffy, cute, occasionally serious what-if-style story (but not an elseworlds!) If you like the JLA, the JSA, the Teen Titans or Young Justice (not the new one. No, the older one that started in the 90s) they're all here and having some amusing, sometimes absurd adventures.
The tale looks at an event where an aging ray and the magic of Klarion - dum dum dum! - the Witch Boy converge to swap the ages of a good chunk of the superheroes in the world. So you're got the JSA as kids being looked after by Stargirl... er, Starwoman, the Justice League as teens, Young Justice as adults, etc.
It has a lot of promise, but instead of one overarching story, it's a series of loosely associated short stories. It gives the whole thing a somewhat awkward feel. It feels almost more like a montage than an actual full-fledged story.
That being said, most of the stories are reasonably fun. A few of them are referencing things that happened earlier and you just have to sort of accept what they are. But a lot are self-contained, based around the comedic possibilities of not only the age swaps, but also that the characters all have the mentality appropriate to their ages instead of retaining their former mentality.
In that, it's a pretty fun ride. Not the best thing to ever come out of the early Young Justice (one of the most fun teams in comics, to me) but entertaining.