Depending on the version of the anime, like Lupin and co would be younger in Part 1, and Part 2 acknowledges a time jump of 5 years, and Cagliostro is intentionally set later as well (despite there being zero visual indication XD)
For Part 1, I'd say Lupin and Fujiko would be around 25, with Fujiko being up to 6 months older (I always imagined Lupin preferring an older woman, haha), with Jigen a year or two older, 26-27. Even though Part Zero has Jigen be Lupin's contemporary, Lupin would be smart enough to be a year or two ahead, or with Jigen's absences, he could be a year or two behind. Goemon would be the youngest, but again, only by one or two years compared to Lupin/Fujiko, so 23-24. Zenigata would be at least 10-15 years older, so about 35-40 in Part 1.
Then Part Two brings them up by 5 years, so:
Lupin & Fujiko- 30
Jigen- 31/32
Goemon- 28/29
Zenigata- 40-45
I'd probably keep them around that age for all future entries, aside from Cagliostro, where I'd put them another 5 years older, which would probably be enough to make them older and wiser, but not visibly.
That being said, sometimes I think Jigen is the one character who can be made to look a lot older if they draw him with more lines for his cheekbones, like in Voyage to Danger/Orders to Assassinate Lupin