User blog comment:Mobo85/Questions for Aliki Grafft/@comment-4343598-20110909223747/@comment-68.189.187.161-20110910053149

It was stated that Phineas and Ferb were the same age. Seeing how since that was said, only Phineas has had a birthday episode, it's a safe bet that Phineas is the older one (by however few months).