Principal skinner

And Bart and Lisa are still kids and grandpa is still alive, no one tends to age in cartoons apart from maybe very slightly when they do an episode in which it's a characters birthday but even then, no one changes

It's not real life, you know

Barny Gumble still has a functioning liver?

That's because its a floating timeline.
https://en.wikipedia.org/wiki/Floating_timeline

Skinneeer! Is 65 years old now

The Simpsons never age. It's a running gag.

I think it's just different peeple age differently, and were all individual like.
Some Vietnam vets are still 45 if they're just young for their age

it's not the fact that he hasn't aged it's the fact that he's still a Vietnam veteran

Good catch