Let’s address the elephant in the room: No, there are not in fact 104 days of summer vacation for most kids. That’s about 3 and a half months when most U.S. kids get 2 and a half months at best. But we respect Phineas and Ferb for being optimistic, especially with everything the brotherly pair accomplished during their time off.
However, even in-universe the characters knew summer was going on just a bit too long, as many Phineas and Ferb episodes poked fun at the show going on for more than 104 episodes. While it seemed like summer would never end for Phineas Flynn (Vincent Martella) and Ferb Fletcher (Thomas Brodie-Sangster), the show did finally end in 2015 after an 8 year run. But across all 129 episodes and two TV movies, one question still remains: Exactly how old are Phineas and Ferb during all these adventures?
While there’s no exact age ever stated throughout the show’s run, it’s safe to assume certain age milestones for the characters. We know Phineas and Ferb are at least elementary school-aged, as we see many flashbacks to them as babies and toddlers. We also know the boys and their friends cannot be any older than 16, as they are not supposed to be able to drive, though that never limited them if the project required a driver. More importantly, their older sister Candace (Ashley Tisdale) has to take her driver’s test in an early season 1 episode, so we can guess that she is newly 16 or 17, as that is when most U.S. teens take their driving tests.
Now, between the ages of 6 and 15 is still a broad range. Thankfully, this is where the season 4 episode “Act Your Age” comes in. The second to last episode of the original series, “Act Your Age” is a flash forward episode, where the gang we’ve grown to love is aged up by a decade. Instead of building roller coasters and travelling into space, Phineas is instead trying to figure out where he’s going to college at the end of summer. Throughout the episode, we discover many of the group’s future plans, such as Baljeet (Maulik Pancholy) having already graduated high school and college early, and Ferb going abroad to his home country of London for school.
While this is easily the best episode in the show because Phineas and Isabella (Alyson Stoner) officially admit their crushes on each other, it is also the episode that unofficially confirms the gang’s ages. Typically in the U.S., high school students are going to college when they’re 17 or 18. Therefore, if this episode is canonically 10 years in the future, during that crazy summer, Phineas and Ferb are most likely between the ages of 7 and 8. More specifically, Phineas is most likely 7 and turns 8 during the show’s run, as there is an episode where he celebrates his birthday, while Ferb is either 7 or 8 depending on his birthday.
That being said, Phineas and Ferb are getting a revival on Disney+ in the near future with original creators Dan Povenmire and Jeff “Swampy” Marsh. So hopefully the new series will confirm the crazy idea that a bunch of 7 and 8 year olds managed to build all the things Phineas and Ferb did throughout the original show.