I would think that the technology for cryo freezing people would have been newish by 1999, so there probably aren’t too many people from before then who woke up in 3000. There may have been a few people who froze and unfroze around the same time, so he may not be THE oldest, but I think that he’s the oldest person in the series
So when you’re frozen you don’t age. So when Fry goes into the freezer doodle at 25, he also exits at age 25.
For example when Fry, Farnsworth & Bender time travel and loop back around, we wouldn’t give them a negative age. If they spend a year time traveling, we would add a year to their age.
So I think we need to consider the oldest as someone with the most years doing something. However since time travel is involved it’s time spend doing things that they then remember.
So Bender, ignoring for a moment he is a robot, being stuck in Roswell would make him quite old since he remembers it all.
Now let’s talk about Fry specifically. In the season 7 finale, “Meanwhile” Fry plays with the time button, gaining new knowledge every time he pushes the button. Eventually Fry jumps off a building, resetting time once he sees Leela. Fry falls for a long time, but we don’t know how long he falls. Time stops, Fry & Leela live a life, but Farnsworth resets time back to when time first stopped. BUT that means Fry still remembers jumping off the building and ALL the times he pushed the button. As a quick throwaway joke when we resume season 8 we learn 10 years have passed. That means Fry remembers pushing the button to reset time FOR 10 YEARS.
So while Fry may not be the oldest, he is at least 10 years older in terms of memory, which I think is interesting to think about.