Yeah, all of the Fab Five Teen Titans (Dick, Donna, Wally, Roy and Garth) should be in their late 20s now.
About JSA, most of the team is legacy characters these days and the originals are mostly deceased (I think Johnny Thunder just has dementia?). There's five exceptions. Alan is functionally immortal due to the Starheart (which is GL Corps tech), Jay's aging is slowed by the Speed Force (again, actually a technological explanation), and Wildcat is indeed a magical reason, he has nine lives. If he loses them all in quick succession he's dead, but they do regenerate, effectively making him immortal as well. The other two, of course, are Doctor Fate and Spectre, but despite their immortality even they've had legacy characters.