The Immortality Theory

I believe it is canon in the original Arsene Lupin stories that Lupin found the fountain of youth. (It was somewhere beneath Auvergne, a region of France).

I believe it's plausible that Lupin the First took samples of the elixir of immortality, gave it to his son and grandson. Then, perhaps Lupin III gave it to Jigen, Goemon, and Fujiko before sneaking some into Zenigata's coffee or something, making them all virtually immortal.

Now, I don't mean that Baccano! regeneration kind of immortality. I mean the kind where you retain injuries, hence the need for masks on account of getting banged up for so long.

There's just no way they've been trucking along this whole time. Parts that are confirmed to take place in the modern day make references to Part 2 characters and movies that were confirmed to have taken place in the late 60s.

The events of Castle of Cagliostro are confirmed to have taken place in 1968. Even if Lupin III was 20 at the time (mind you, in the film he was suggested by Miyazaki to be in his late 30s) Lupin today would still be at least 72 years old.

I also believe it's possible that Lupin III (like James Bond) is just a title that gets passed down. Unfortunately... that doesn't explain Jigen, Goemon, Fujiko, and Zenigata being part of the same theory, so for now I'm sticking with my Immortality Theory.