Because they're cartoon characters. Some cartoon characters age, others, such as the Simpsons or the characters of Family Guy or Crayon Shin-chan or Sazae-san, don't age at all and sometimes have the times of past events changed to keep their ages consistent. 'Lupin III' chooses to go down this route. Additionally, like Sherlock Holmes, Lupin can be placed in any setting without actually changing.
Furthermore, several times in the franchise the characters have shown to be aware that they are in a cartoon, such as Jigen grumbling about how unbelievable "King Tut" seemingly floating is in Part 2 Episode 7.
