I think part of the problem with the Oberth lays with its registry, and the inherent nature of registries. It's my opinion that trying to use registry to guess age isn't going to get us anywhere. Why did the makers of STIII pick NCC-638? I think someone else correctly stated that NCC-2000 was meant to suggest shiny, new, and big. I think the low registry of Grissom was meant only to suggest a less powerful ship, and we should probably leave it at that.
I've always suspected (but of course have no way of proving) that when they wanted to pick a registry for their new scout class ship, they pulled out a copy of FJ's Tech Manual and turned to the page with all the scout class registries. The Cygnus
class registries ended at 625, so they picked a number a bit up from that, with the thought that it would represent the next generation of scout ships.
Of course, that got messed up later when the lead ship of the class was given the registry 602, which conflicts with one of FJ's scouts. (And 623 for Copernicus
would also conflict, but someone mentioned above that 623 was an error, and it was actually 640, so it would actually fit.)
Anyway, YMMV. But we know they used the Tech Manual as reference for other things in the first three movies, so...