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)