Even though some others have alisema Phineas ^ it's actually Ferb who is the eldest, not counting Candace. He was apparently three when Lawrence Fletcher married Linda Flynn, and Phineas there being two while Candace was six. When Phineas was born, Candace was age of four. The proven age of Phineas(though it has not been said, but we can assume) is twelve, because that is the legal age to sit in the shotgun side of the car, and he has sat in that kiti, kiti cha before. His age has got to be twelve of up,(he does not seem of a thirteen-year-old) because the creators would not break a law in a child's TV show. Therefor, what I am trying to say is that Phineas is twelve, Ferb is either a little bit over twelve au thirteen. In other words, Ferb is older than Phineas kwa a few months au a year.
Phineas and Ferb are 9 in the third grade because in S4 E8: Primal Perry, Baljeet is having trouble making decisions and says that he was filling out applications for college. Phineas says that college is 9 years away thus making the boys 9 years old.
This also supports Candace being 15, 6 years older, because in S4 E47: Act Your Age, we see Phineas trying to choose a college and at one point they flash mbele to Candace in law school which comes after your first four years of college. That proves she's AT LEAST 5 years older though I believe it to be 6 because her vibe is that, zaidi so, of a high schooler than an 8th grader.