Cartoon characters have a huge advantage over their live-action counterparts. And that's the fact that they can go years and years without ever actually aging. To paraphrase Matthew McConaughey, we get older, but they stay the same age. But if they did age, we would suddenly view these characters quite differently. For instance, if Doug and Patti from Doug had aged normally, instead of being 11 or 12, they would be 37.
On King of the Hill,Bobby starts out as an 11-year-old when the show began in 1997. By the time the show went off the air in 2010, Bobby gained three years and was now 14. But if he had aged normally, he'd now be a 31-year-old, and hopefully would be entertaining people with his unique brand of prop comedy.
On The Rugrats, the kids were all, well, rugrats when the show began in 1990. Then on All Grown Upstarting in 2003, the kids all were aged up to pre-teens...but it wasn't a perfect chronological match. In 2017, Tommy should be 27, Chuckie should be 28, and Angelica should be 29.
Tiny Toons began in 1990, featuring characters who were supposed to be teenage equivalents of classic Looney Toons characters, such as Buster and Babs Bunny as younger versions of Bugs. Babs once revealed that she was 14 years old, and since all the characters appeared to be the same age, we can only assume that today they would all be around 41 or so. Which would probably make them older than the supposed age that Bugs is supposed to be.
Futurama mostly took place in the year 3000...although Fry was initially cryogenically frozen at age 25 as the calendar turned from 1999 to 2000. So in 2017, he would be 42, and if it was 3017 in Futurama world, he would be 1,042. Meanwhile, Leela was also 25 at the start of Futurama, and would be 42 in 3017.
When Bob's Burgers began in 2011, Tina was 13, Gene 11, and Louise 9. Which means that, by 2017, Tina should be out of high school, Gene should be getting ready to graduate, and Louise would on the verge of getting her driver's license. And the thought of Louise driving around in a car kind of makes us want these characters to actually become their proper ages.
Goofy's son initially was named Goofy, Jr. in shorts from the 1950s. But if we ignore that and just focus on the modern version of Max Goof, he was 11 years old when Goof Troop came out in 1992. These days, Max would be a 36-year-old, perhaps with a son of his own.
On Kim Possible, the characters broke from cartoon tradition and actually did age from year to year. On the series finale from 2007, Kim was graduating high school, which would have made her 17 or 18 at the time. In 2017, she would be 28 or so.
It's not entirely clear the exact ages of each Muppet Baby on the show Muppet Babies, but since they're babies we have to assume they're anywhere from 0-2 years old. Since the show premiered in 1984, that would mean that the babies would now be adults who are age 33-35. Which is weird, because the adult version of The Muppets who started out on The Muppet Show would now be much older than themselves.
Blossom, Bubbles and Buttercup are kindergartners at age five on the show when it premiered in 1998. And they remained the same age when Powerpuff Girls was brought back in 2016. But if they had aged normally, they would now be about 24...even if they had some sort of special power that keeps them the exact same size.
Bart and Lisa Simpson have always been 10 and 8 years old, respectively, going back to the premiere of The Simpsons in 1989. It's now 28 years later, which means Bart should really be 38 and Lisa 36. To put thing into even greater perspective, Homer Simpson is believed to be anywhere between 34 and 39 (despite going to high school in the 1970s).
In the fourth season of South Park, the show did something that you don't see on most other cartoons...they had the kids actually age, and advance from the third grade to the fourth grade. However, since then, Kyle, Stan, Kenny and Cartman have remained at ten years old. But if they had continued to age since the show started in 1997, they would now be adults about to turn the big 3-0.
When The Flintstones began, Fred was a relatively young man at age 31. If you take the theme's song word for it, the show was set in the Stone Age, which would mean that Fred would now be about 2.5 million years old...plus 31. However, that math goes out the window if you believe The Flintstones actually takes place in a post-apocalyptic future, not the past.