Phineas Flynn (voiced by Vincent Martella) Like other characters, his age is not mentioned. Phineas, along with his stepbrother Ferb Fletcher, star in the A-Plot of every episode. The series concerns Phineas's attempts to avoid boredom by finding something new to do every day of the summer vacation. He does this with his less-talkative stepbrother Ferb, and often with many other neighborhood children. He is known to be very selfless and energetic. Phineas has many catch phrases like "Oh, there you are, Perry" or "Hey, Ferb! I know what we're going to do today!" and "Hey, where's Perry?".
Source: Wikipedia, "List of Phineas and Ferb characters", available under the CC-BY-SA License.