We don't know yet, but the upcoming movie may shed some light on Perry's past.
7 years old.
ferb is eleven and Phineas is eight
In the episode 'Interview with a platypus' it shows Phineas playing music as a toddler, along with young Perry. So it could be possible he is around the same age as Phineas, around 10 years old.
79
She is 15
8
Phineas (Vincent Martella) is 17 and Ferb (Thomas Sangster) is 19. But in the show they are probally 10 or 11.
im pretty sure she is as old as phineas who it probably 9 or 10
16
probably about ten years old
approximately 11.
15 in s1 and 16 in s2