He's got good range, so a deeper or older voice isn't a problem for him.
He actually said (in two separate interviews afaik) that, when they started recording ED (first thing they ever did), they still hadn't decided Noctis' age. Which is either a misunderstanding or something very weird, as young Noctis doesn't really look like a guy that could be in his 30s. And yeah, I think he said that he grows also in how his voice changes and he definitely has range. He seems like a great guy, and I'm really liking his acting in XV (haven't seen him in anything else, I think). Still playing in Japanese, at least the first time.
Actually, talking about this and considering this is a thread for the story and all that: have you guys noticed that Old Noctis doesn't have red eyes during Chapter 0? Or maybe I'm blind (the intense heat that deforms visibility doesn't certainly help), but it seems like he has his regular blue ones.
So maybe he doesn't permanently have them like in the CG render, or that happens later on. Perhaps when he uses the exploding ring after sitting in his throne (which would make sense, as he appears, in the render, with the formal suit and not the King cloak).
Edit: actually, Chapter 0 is him
without the ring. So I'm guessing that this trailer scenes with him on the throne are after fighting Ifrit, as it seemed.