User blog comment:Poptropica411/Phineas plus Isabella equals Phineas and Ferb: Across the Second Dimension in Fabulous 2D?/@comment-3329115-20110213211355/@comment-1478533-20110224051430

Well, they said Phineas and Ferb have been the same age. But there is an episode about Phineas' birthday coming up; since we've seen nothing about Ferb's birthday, Phineas might be the older one.