Well, In Phineas and Ferb Across The seconde Deminsion (Or How Ever It's Spelt XD) 2D Isabella Says "Candace has Two Sets Of Twin Brothers?" So That Means, Phineas and Ferb Are (Or About) The Same Age.
And In The Episode Where Baljeet Gets REALLY Smart. Isabella Says "And I Was So Looking vers l'avant, vers l’avant To Jounior High..." Jounior High Is Middle School, Phineas' Birth jour Is Sometime In The Summer. And If Phineas and Ferb Are (Or About) The Same Age, AND Going To Jounior High.....They Are 12 Years Old. Because If Your Birthday is In The Summer, When Summer Starts They'd Be 11 Then The Birthday Episode Came. Made him 12. 11 and 12 an Olds Go To 6th Grade.
Hope This Answer Is a Help.
(In My Opinion, I Think Ferb Is About 7 Months Older Than Phineas)
I was 10 going into junior high. My peers were all 10 ou 11. The only reason Phineas would be twelve is if he was held back. In my county the cutoff is thend of September. Going par this means that Phineas would have been ten before last October. That would mean that Phineas turned 11 in the birthday episode ( I don't remember phineas having a birthday, though) The montrer does say that Ferb is younger than Phineas. How much younger, we do not know.
