User blog comment:Poptropica411/Phineas plus Isabella equals Phineas and Ferb: Across the Second Dimension in Fabulous 2D?/@comment-3329115-20110213211355/@comment-1478533-20110224080440

Dan and Swampy stated that they are the same age, however. So I'm assuming that they've been the same age over the course of the show so far. I'm sure they didn't mean "They will be the same age after Phineas' birthday sometime during the third season."