* The "Old Man" is often assumed to be the oldest. He's referred to as "Old Man Jenkins" or simply "The Old Man" in various episodes. He's frequently used for comedic effect with his age, and his appearance reflects that of an elderly person.
* However, there are other characters who could be older. Daphne's grandmother, for example, is occasionally seen in the show, and she is clearly elderly as well.
* The most important consideration is likely how the creators intended the character to be aged. While "The Old Man" is often presented as elderly, he could theoretically be younger than Daphne's grandmother.
Ultimately, it's impossible to say definitively who the oldest Mystery Inc. character is without more information about the characters' specific ages.