Carrie Fisher is only 58? That would make her 19 when she filmed SW: ANH. I haven’t seen those movies in a loooong time, but I imagine her being older than that in the film - say closer to 29. (Of course, I was only eight in '77 when I saw it; she could have been 12 and I would have pictured her as being “older.”)
Of course, if she’s fibbing about her age and she’s really older, that only mean she looks better. Either way, I’d say her looks are a testament to decades of clean, sober living.