Is it normal that bart, lisa, and maggie simpson never grow up??

That's true but how do you pin a timeline to something that exists only discontinuously. The Simpsons may have existed for a quarter of a century but only for 23 minutes of every week. And they show old ones, plus people in different countries see them in differently spaced intervals. You could artificially pin it to the present day on the day each episode is made, except it takes longer to make an episode than to show it. Another paradigm shift in terms of chronology.

The only way to avoid tying yourself up in knots is to assume a very compressed timeline, which is what The Simpsons do. The characters actually do grow older (I remember Bart having a birthday) but they grow older at a vastly retarded rate. This allows the makers to deal with issues such as birthdays and rites of passage without getting bogged down in all the problems of "real-time".

I think most of us are familiar enough with the medium to not expect the characters to age, in the same way Wile E. Coyote always recovers from an accident in next to no time. It's a device that is a requirement of the medium. It's fine to ask why characters don't age, but there's a good reason why it's easier if they don't.

Personally, I do fall on your side of the argument sometimes as I like it when cartoon makers subvert the usual rules of the medium. I distinctly remember a Tom and Jerry where Jerry shot the fur off the top of Tom's head and in the next scene Tom wore an awful ginger wig. The clever part is that he wore it right through to the end of the show, even though it had already served its purpose as a comedic device.