They don't have ages, for sure. The people that made it, wanted to leave there ages not-answers. So, they gave them the age of less then fifteen, they are over five as they got Perry five years ago, and they were already born, so they are between the ages of six and fourteen. They seem like they between seven and twelve. Seven from 'Act your age', which is ten years in the future, they are seventeen or eighteen, which, if they are seventeen in ten years, they would be seven. If they were eighteen in that one, then they would be eight. Phineas was said to be nine, when the show started, and Phineas and Ferb are the same age. In another one, they are said to be 30 in 20 years, so they would be ten. Phineas has been shown to ride in the front seat of a car, which you have to be twelve to do. But I think when the it came out, you only had to be 11 in some states.