Not that old. Going by most canon, she's as old as any other character, unless you want to spot her spending a lot of time either as a Female Fury before meeting Scott or after Scott's escape, which Byrne does in his retcon, but only a few hundred years' worth (which is still bullshit, because that's a few hundred years of Barda following Darkseid's orders after having the moral reawakening Kirby depicted. Yeah, don't think so).